Political debate: Do not adjust your set

General elections will be transformed by an historic agreement reached last night under which Gordon Brown, David Cameron and Nick Clegg will face questions from voters in three 90-minute live television debates.

The three main parties admitted there would be no going back at future elections as they signed a 76-point agreement with the broadcasters for US-style debates which look certain to dominate the imminent election campaign.

Although dates will not be finalised until the election date is announced, The Independent has learnt that they have been pencilled in for 8pm to 9.30pm on Thursdays 15 and 22 April and Wednesday 28 April. Polling day now looks certain to be on 6 May. One reason for Mr Brown delaying the election, he has told aides, is to give him time to prepare for the debates.

Talks will now open on staging television debates between other frontbenchers – starting with one between the Chancellor Alistair Darling, his Tory shadow George Osborne and Vince Cable, the Liberal Democrats' Treasury spokesman. All parties insist they want a wider range. The complexity of the arrangements agreed for each of the debates is reflected in a detailed list, covering everything from the make-up of the audience and time limits on answers to dealing with topical stories and ensuring the three leaders finish by shaking hands.

Most of the 200-strong audience at each debate will be picked by pollsters ICM to ensure a balance of gender, age, ethnicity, social class and voting intention. They will be asked to applaud only at the start and end of the programme. Broadcasters and the parties settled the timing of the debates by drawing lots. The first, in Manchester, will be shown on ITV1 and hosted by Alastair Stewart. It will focus on domestic politics including the NHS, education, immigration and trust in politics.

The second, broadcast by Sky News from Bristol, will be chaired by its political editor Adam Boulton and cover foreign affairs – including Afghanistan, Iraq and Europe. The final one, in Birmingham, will be screened on BBC1 and be chaired by the veteran Question Time host David Dimbleby. It will cover economic issues including the public deficit, tax, public services, recession and the banks.

Each politician will – after giving a short reaction to any "major" national or global story of the day – have one minute to make an opening statement on the main theme. Questions – pre-picked by broadcasters' panels from those put forward by audience members and sent by email – will then be put to the leaders, who will not see them in advance.

Each will have a minute to answer, then another minute to respond to the others' answers followed by up to another four minutes of "open debate". The second half will be dedicated to more general questions.

Last night's deal follows months of tortuous negotiations after Mr Brown, trailing badly in the opinion polls last autumn, took up the challenge of his two rivals. Attempts to secure debates at previous elections foundered when talks on the fine detail became bogged down. This time the controversy over MPs' expenses meant no party could block a chance to re-engage with a disenchanted public.

The Liberal Democrats are widely seen as the biggest beneficiary from last night's breakthrough. Although broadcasters must give them equal airtime once an election is called, Mr Clegg now has a huge platform that his predecessors would have envied. His party won 22 per cent of the votes in 2005 and its poll rating normally rises during a campaign – an effect that Mr Clegg hopes will be enhanced by the TV debates.

Party leaders welcomed the deal, which broadcasters hailed as a "game-changer". Privately, some party officials fear the debates will distort the entire campaign, with many hours spent "prepping" the leaders, rehearsing their lines and working out how to avoid the "gaffes" that commentators will be eagerly awaiting. Some predict three closely fought contests rather than a knockout blow or disastrous mistake.

Both Labour and the Tories have already called in advisers to Barack Obama's presidential campaign for advice on how to handle the upcoming television debates. But the presence of the third leader adds a different dimension to the British version. "Clegg is the wild card," one Tory source admitted.

Mr Cameron, who was first to call for the TV debates in May 2007, said he was "absolutely delighted". He added: "I hope these debates will play a part in helping to restore trust in politics. We will have the chance to talk to millions of people in our country who are fed up with politics, fed up with politicians, think we're all the same and think nothing changes."

The Prime Minister said: "It is the country's decision and I want to be a part of that debate. I am so optimistic about this country's future that I want to debate the big issues and I want to show that we are best for jobs, for the health service, for the public services as a whole, for tackling antisocial behaviour and for dealing with the economy."

Mr Clegg said: "These debates will be the centrepiece of the general election campaign and are a huge opportunity to reconnect people with politics. It's great news that at a time when trust in politicians is at a low people will be able to have a good look at the party leaders and find out what they stand for."

The Tories have signed up Squier, Knapp, Dunn Communications, a Washington-based political consultancy. Anita Dunn is a former White House communications director and Bill Knapp has worked on four Democratic Party presidential campaigns.

Labour's preparations are being helped by Benenson Strategy Group run by Joel Benenson, who advised Mr Obama on his debates with John McCain, and David Axelrod, who was chief campaign consultant to the Obama campaign. America saw its first presidential debate 50 years ago. Debates have been held in Australia since 1984; in Canada regularly since 1979 after a one-off in 1968 and election debates are staged in most European countries.

Anchors away! The men in the fourth hot seat

Alastair Stewart

A familiar face on ITV news programmes since the 1980s, Alastair Stewart OBE has been co-presenter of ITV's 6.30pm bulletin since September last year. Those in the South-east will probably know him best for his 16 years as the host of London Tonight. Outside the capital, the 58-year-old is instantly recognisable as the voice of, quite literally, car crash TV. He hosted Police Camera Action! on ITV, which shows archived footage of road crimes filmed from police cars and helicopters, but was forced to resign after his second conviction for drink driving in 2003. A former war correspondent, he hosted ITV's election night coverage in 2005 as well as the 2008 US presidential election.

David Dimbleby

The perennial face of election night, 71-year-old Dimbleby has anchored the BBC's coverage of every general election since 1979. He has covered many landmark events, including the funeral of Diana, Princess of Wales, and conducted several set-piece interviews, including with Bill Clinton. He has been the host of BBC's flagship political discussion programme Question Time since 1994, perhaps most notably when the BNP's Nick Griffin appeared on the panel in October last year. In the past 16 years only an attack by a bullock on his Sussex farm in November last year has kept him from the chair.

Adam Boulton

Sky News spearheaded the campaign for the leaders debates, and Adam Boulton has been the channel's political editor since its launch in 1989. He presents Sky's Sunday Live with Adam Boulton and covers Prime Minister's Questions. His first book, Tony's Ten Years, was published in 2008. He is married to Blair's former spin-doctor Anji Hunter, and guests at their wedding in 2006 included the leading Blairites David Blunkett and Peter Hain. Memorably described by Andrew Marr as a "genuine political nutter", Boulton retorted that he had pioneered the "character political commentary" and that Marr "would not exist" without him. Boulton has interviewed George W Bush, Barack Obama, and every prime minister from Alec Douglas-Home onwards.