Gordon Brown left No 10 for Buckingham Palace today to ask the Queen for a dissolution of parliament before a May 6 General Election.

His convoy swept out of Downing Street after a final Cabinet session ahead of what opinion polls say will be the closest contest for nearly 20 years.

On his return, the Prime Minister will officially announce the date, before all three party leaders hit the campaign trail.

Earlier David Cameron said the country would be facing a "big choice" at the ballot box.

"I think the Conservatives - the modern Conservatives - have got the energy, the leadership, the values, to get things done in our country and that is what we need - a fresh start," the Tory leader told reporters outside his west London home.

Liberal Democrat Leader Nick Clegg struck an upbeat note as he set off from his home in Putney, south west London, insisting that the contest would not simply be a two-way fight between Labour and the Tories.

"This is a choice now between the old politics of the two old parties and something new, something different, which the Liberal Democrats offer," he said.

"This is not a two-horse race between the two old parties, Labour and the Conservatives.

"People have got a real choice this time and I think that's why this election is wide open. All bets are off."

Parliament will be dissolved next Monday. But Mr Brown, accompanied by his wife Sarah, will immediately embark on a "GB On The Road" campaign based around direct contact with voters.

Features adopted from US President Barack Obama's successful run for the White House will include meeting workers in their canteens and having personal chats with people in their homes.

Heading initially to crucial seats in the South East, the Prime Minister's principal theme will be to warn voters that they cannot trust the Conservatives with the economy.

Mr Cameron's message will be that he is fighting to give hope to "the Great Ignored".

He told reporters that the question facing the country was: "Do we want five more years of Gordon Brown? Do you want change with the Conservatives who have got the energy to get this country moving?"

With the election result more uncertain at this point in the campaign than at any election since 1992, the parties are digging in for what is likely to be a bitter slog to the end.

While two polls today gave the Tories a 10-point lead - the kind of advantage they need to form a majority government - another suggested Labour were just four points behind.

That would almost certainly result in a hung Parliament and may even give Labour the most seats. Labour aides said last night that they were fighting to win.