The Prime Minister will face his Conservative and Liberal Democrat opponents on three separate occasions, on the BBC, ITV and Sky in an attempt to bring X factor appeal to political discourse.

The news, which brought an instant complaint of bias from the SNP, followed months of behind-the-scenes negotiations between party officials and a joint broadcasting team.

The three leaders will be subjected to a total of around four-and-a-half hours of live television scrutiny. ITV will host the first debate, which will be presented by Alistair Stewart. Sky News will go second with political editor Adam Boulton in the chair. The third and final debate, on the BBC, will be fronted by Question Time presenter David Dimbleby.

Leaders will appear in front of an audience for about 90 minutes, but the details of the format have not been agreed.

The debates will signal a transformation of the British General Election campaign which has over the past decade been heading from the battlebus and community hall to the TV studio.

Live presidential debates in the US and other countries have proved to be the highlight of election campaigns and are seen as having raised voters’ interest. Opposition leaders regularly call for them but Prime Ministers have never in the past surrendered the advantages of incumbency to grant them.

However, with Mr Brown trailing so far in the polls Labour strategists feel he has nothing to lose in open debate with Mr Cameron and Mr Clegg.

Last night Mr Cameron, who has been calling for a TV debate for some time, described the move as "a thoroughly good thing that will enliven our democracy". He added: "It is a step forward in such a bad year for our parliament."

The Labour Party is understood to be happy the debates have been agreed although it would rather they had started earlier in the election campaign.

LibDem leader Mr Clegg, who finds his party squeezed at election time, was the most delighted. He said: "After a terrible year for politicians because of the expenses scandals, these debates will be an opportunity to start re-engaging people with politics. Next year everybody will get the chance to decide this

country’s future. I hope an open, honest and vigorous debate will encourage more people to have their say at the ballot box."

SNP First Minister Alex Salmond said it was entirely unacceptable for the broad-

casters to exclude the party that forms the government of Scotland. He said: "If these debates are to be at all relevant to their audiences, they must reflect the democratic reality of Scotland and political diversity across the UK. And that must include SNP involvement in debates broadcast in Scotland."

He added: "The broadcasters have got to meet their public service obligations to audiences across the UK, and for them to propose debates which signally fail to do so shows an extraordinarily high-handed attitude and depressingly metropolitan mindset."

The broadcasters each said they would be taking steps to see that the other parties, including those in Scotland, Wales and Northern Ireland, received proper coverage in line with obligations for due impartiality.

Party leader debates are a regular feature of Scottish parliamentary elections. The BBC said it would be holding separate debates among all the main parties, which would be broadcast on BBC Scotland, Wales and Northern Ireland and across the UK on the BBC News Channel.

Labour’s Scottish Secretary, Jim Murphy, reissued his challenge to Scottish party leaders at Westminster to join him in a televised debate during the General Election campaign. Mr Murphy’s invitation was specifically to Angus Robertson, the SNP

Westminster leader, as part of Labour’s attempt to sideline Mr Salmond in the campaign.

Details of the TV debates came as a Comres poll for the Independent showed the Conservative lead over Labour slipping by one point to 38%, with Labour up two points to 29%. The figures would leave the Tories five seats short of an overall majority. The LibDems scored 19%, down one, and others rated at 14%, down two points.

Responses to the survey of 1000 voters appeared to indicate that Mr Cameron has failed to seal the deal with the electorate. By 52% to 44%, voters agreed with the statement that "a Conservative government would mainly represent the interests of the well-off rather than the

ordinary people".

This could mean that Labour’s message to its core voters is getting through but the public remained divided -- 47% each way -- on whether Labour or the Conservatives would better defend public services. In the marginal seats that will decide the election the Conservatives are confident they will win the argument for change.