'There hasn't been anything like it on TV since before John Logie Baird,' ran the joke.

For 12 years the BBC operated Britain's sole television service and for another eight it competed with ITV, then by April 1964 the BBC was ready to launch its second channel and the nation's third. Buoyed by the government's Pilkington Report and restoration of the licence fee to a realistic level, and pressed by TV manufacturers eager to market new technology sets, the BBC determined to produce a channel of cultural distinction, pursuing excellence in drama, comedy, sciences and the arts.

The new station - to be introduced in London and the Home Counties before unfolding nationwide - was the subject of great expectation. Dennis Potter declared in his Daily Herald column, 'BBC2 offers us, for the first time, a genuinely planned alternative instead of a haphazard chase up the ratings table.' Writing in Radio Times, the BBC's Director of Television Kenneth Adam requested 'recognition of what we are seeking to do, and forbearance when we fall short of our intention.' Yet none could have foreseen how this falling short would blight even the triumphal Opening Night.

Tuesday 20 April 1964 was the launch date, when what had always been simply 'BBC television' became BBC1 in order to accommodate the new arrival. BBC2 was set to open at 7.20pm with a ten-minute Line-Up, and then screen eccentric comedy from the Alberts and from the Soviet Union's number one standup, Arkady Raikin; Howard Keel in a new BBC recording of Cole Porter's Kiss Me, Kate; and Off With a Bang, an ambitious fireworks display from Southend. Swiss Television had even arranged to relay the entire evening by closed-circuit to delegates at the annual Montreux TV festival.

Michael Peacock - BBC2's unflappable Chief of Programmes, young at 34 - was steering the ship from his sixth-floor office at Television Centre in White City, West London, just as he had through two years of planning. This was the Big Night. On the dot of 6pm, uniformed catering staff wheeled in a trolley the content of which had been carefully specified: aperitifs, ice, glasses for 12, cigars, cigarettes, two Thermos of soup, cold meats, salads, cheeses, fruit and coffee. A dozen bottles of champagne were ordered for 11 o'clock, when the new channel's first night would close down.

Numerous other key figures in BBC2's start were in the Club bar, toasting the night ahead. Reflecting on subsequent events in Ariel, the Corporation's staff journal, Line-Up producer Rowan Ayers saw the evening as the culmination of a 'years-old dream of getting BBC2 off the drawing-board and on to the air,' adding, 'it seemed at Zero minus 60 minutes little short of gilt-edged.' Then, at 6.34pm, the lights began to flicker. 'Too many people are switching on to watch!' one wag remarked. Then the lights failed completely.

A serious electrical fault had just developed at Battersea Power Station, five miles south of Television Centre. Switches to isolate the problem had burst into flames, and the fire then swept through cable ducts. At precisely the same time, out at Iver, Buckinghamshire, there was a breakdown in a 60,000-volt cable that fed electricity south from the Midlands. These twin failures caused an extensive blackout through west London and in parts of the central area.

The Battersea Power Station phone system was paralysed and firemen were not summoned for a full hour, until a worker drove to a phone box and dialled 999. There was an irony here. During summer 1963, when BBC2's Opening Night plans were being formulated, the Head of Light Entertainment, Tom Sloan, had suggested placing camera crews across London to demonstrate the working capital, basing one of them at Battersea Power Station. The idea had been rejected.

It took 60 fire-fighters more than two hours to extinguish the Battersea inferno. Central Line trains ground to a halt, theatre performances were cancelled, cinemas closed, a QPR football match was abandoned, hospitals were affected, as were Buckingham Palace, the House of Commons and several thousand homes. The Prime Minister, it was reported, ate by candlelight at a National Liberal Club dinner at the Caf� Royal, and Lena Horne sang in twilight at the Palladium. A man working on the new underground Victoria Line fell down a 30-foot shaft at Vauxhall and was seriously injured. Though localised generators soon helped get life moving again, it would be more than six hours before Battersea was providing output.

When the power failed it was dusk, not dark. In the International Communications Area at Television Centre a group of Fleet Street writers were gathered to watch BBC2's opening night. Candles, storm lanterns and bonhomie were quickly located. On the sixth floor, though, Michael Peacock was watching the clock. 'There was a general preoccupation with when the power would come back,' he remembers. 'After all, when you lose power it comes back, it doesn't go away for a long time.' The night's events would prove him wrong.

Though there was emergency lighting for the stairs, Television Centre wasn't equipped with a generator of sufficient output to sustain the studios. 'To power up the whole of the building - six or seven studios as well as the central distribution system - would have needed a bloody big generator,' Peacock reflects.

It wasn't only BBC2's opening that was in disarray either, for several BBC1 productions, including Compact, Z Cars and This is Your Life, were meant to have been recorded in the studios that night, and the evening's BBC1 transmission schedule was severely affected - four programmes were lost, replaced by apologetic caption cards and the western movie Devil's Canyon. The signals were beamed to the Crystal Palace transmitter from the BBC's former HQ at Alexandra Palace in north London, an area unaffected by the Battersea fiasco.

Moreover, the old studios were also helping out on BBC2, for the commonly held view that nothing went out on the second channel's first evening is wrong.

'Back home again, that Yorkshire bus-conductress who was sacked last week for calling Pakistani passengers "Stinking w***" has got her job back. Union representatives went to see the management and the conductress made an apology.'

These, extraordinarily, were the first words heard on BBC2 on its launch night. Because of the west London power failure, the newsrooms, based in the BBC's original 1930s TV studios at Alexandra Palace, were going live when they least expected. Seated at a small desk in Studio A, the journalist Gerald Priestland found himself reading a bulletin on BBC2 plum on its scheduled start at 7.20. Even here gremlins were at work, because for the first two minutes no sound accompanied the vision; it finally burst through on the bus-conductress story.

This much is known because by extraordinary good fortune a 14-minute video of that first-night news bulletin was discovered last year in a box of obsolete tapes out at Kingswood Warren, near Tadworth, Surrey. In turn a private house, finishing school and hotel, this gothic pile was acquired by the BBC in 1948 for its important Research and Development operations. Recently screened to members of the Test Card Circle, and also at the National Film Theatre as part of the annual Missing Believed Wiped TV festival, the video is a deliciously hilarious example of antique broadcasting.

Gerald Priestland is sitting at a desk on which is placed a portable manual typewriter and standard 1960s telephone, and towards which is pointed a microphone on a stand. Behind him, other newsroom staff and journalists contemplate documents. A lanky man with swept-high hair, a knotted brow and a casual sweater over his tie, Priestland sits bolt upright, and when the director rings the phone and interrupts his flow - as he does on three occasions in these first few minutes - the inescapable impression is of John Cleese in 'And now for something completely different' guise.

Mid-evening, back at Television Centre, the emergency lighting was failing to pierce the enveloping gloom. As Rowan Ayers would report, candles and torches met in confused pools of light in corridors and offices, the Line-Up presenters interviewed one another to pass the time, and engineers opened a book on when the power would come back. Michael Peacock was calmly re-drawing his plans by the half-hour. 'As we moved past eight o'clock and towards nine I realised we couldn't have anything like a full first evening,' he remembers. 'We finally decided that if power hadn't come back by 9.45 we'd abandon everything and start BBC2 the following night.'

The London ITV company Rediffusion offered generous use of its control centre and transmitters after 10.30 but it was already too late. The gesture was all the more poignant in the light of how, in September 1955, the BBC had stolen a publicity march by killing off Grace Archer on ITV's opening night. It crossed some minds this evening that ITV had scuppered BBC2's launch by exacting piratical revenge down at Battersea, but this was quickly discounted.

Throughout the long candlelit evening, BBC press officers served a running buffet of drink and information to the band of journalists, all of whom were happy with an unexpected lead story for the following morning - tales of dashed plans, plucky heroism and photographs of Peacock and co with their lanterns. Among BBC staff there was cheering when word came though that Alexandra Palace had beaten ITN with film of the Battersea Power Station fire, screened in the evening's main BBC1 bulletin.

By 10pm, though, the ice-buckets turning rapidly to water, Television Centre was evacuated for the night. Michael Peacock told the assembled reporters, 'We are very disappointed, but we will have a very good night tomorrow.' In an evocative aide-memoire dictated the next morning, Kenneth Adam declared, 'When I got to Reception, after negotiating the stairs with a flickering candle to guide me, I was surrounded by sympathisers, including large numbers of actors released from their vigil - and actresses, two of whom I lifted home in my car. It was like the Blitz all over again.'

The next morning there was indeed much sympathetic front-page coverage. 'All publicity is good publicity...' Daily Mail editor Mike Randall would write to Kenneth Adam. The Director of Television himself was so ecstatic at how Alexandra Palace staff had kept their heads during the crisis that he wrote to the News Editor, Waldo Maguire, proclaiming, 'Heroic action from ancient halls - with Gerald Priestland in magnificent form - cheered and heartened us all in our dimlit tragedy. We salute and thank our latterday saints!' The letter was pinned with pride on Alexandra Palace staff noticeboards.

At 11am, Play School unexpectedly became the first proper broadcast on the BBC's second channel, and that evening's menu was replaced by the postponed first-night schedule, fireworks and all, kicking off with a Line-Up in which presenter Denis Tuohy blew out a flickering candle.

BBC director-general Hugh Carleton Greene expressed himself 'pleased to the point of surprise by the sympathetic and friendly result in the Press after the calamity'. Forty years on, Michael Peacock reflects on how the night's unforeseen events turned emphatically in BBC2's favour. 'The moral of the power failure was "no mistake no publicity, big mistake big publicity,"' he says. 'Had it not happened, the nation may not have known that BBC2 had gone on the air. As it was, everybody knew that we hadn't.'

Mark Lewisohn wishes to acknowledge the BBC Written Archives Centre for providing information.