LONDON (AFP) - British Prime Minister David Cameron will on Monday ask Queen Elizabeth to dissolve parliament, kicking off formal campaigning for one of the most unpredictable elections in decades.

Both Conservative leader Cameron and opposition Labour leader Ed Miliband have already set out their battle lines for May 7, focusing respectively on the economy and the state-run National Health Service (NHS).

Campaign buses will criss-cross the country and candidates with colourful rosettes from their respective parties will go door-to-door in an election in which every last ballot could be key.

The two main parties concede there is still everything to play for, and much is at stake - not least because Cameron has promised a referendum on Britain's membership of the European Union if he is elected for a second term.

The Conservatives and Labour have been neck and neck in opinion polls for months, and two new surveys published on Sunday indicate the race remains incredibly tight.

A ComRes poll for ITV News and the Daily Mail put the Tories four points ahead on 36 per cent, to Labour's 32 per cent - their biggest advantage since September 2010.

"While this is bound to be a volatile election with both Labour and the Conservatives running each other close, the longer term trends points towards the Conservatives beginning to dig in and strengthen their position," commented Tom Mludzinski, head of political polling at ComRes.

However, a YouGov poll for the Sunday Times gave the exact opposite result, putting Labour four points ahead on 36 per cent.

The surveys were conducted after the first major election event, a live televised question and answer session on Thursday, in which neither Cameron nor Miliband delivered a knock-out blow.

Neither the Tories or Labour however look likely to win a majority in the 650-seat House of Commons, and would have to seek the support of smaller parties to form a stable government.

"There is a real prospect of a messy result that makes coalition-formation exceedingly difficult," YouGov president Peter Kellner wrote in a commentary.

"I would not be surprised if we ended up with a minority single-party government."

- NHS as a battleground -

Cameron will head to Buckingham Palace to ask Queen Elizabeth to formally dissolve parliament, after which a ceremony is held in London's financial hub where a Royal Proclamation calling for a new parliament is read out.

But with the election day having been set in 2010 when Cameron formed a coalition with the centrist Liberal Democrats, the campaign has been simmering for months.

The centre-right Conservatives, who say their austerity programme has steered Britain out of recession and has slashed the deficit, are trying to keep the debate focused on the economy.

In a personal attack on his rivals at the weekend, Cameron condemned Labour as "hopeless, sneering socialists" and said that Miliband was not up to the job of prime minister.

He also pledged to improve the state-run, free-to-access health service that polls indicate is top of voters' concerns, promising to extend weekday services through the weekend to make it a "seven-day NHS".

Labour meanwhile has focused on what it says has been the decline in living standards, and on protecting public services.

Miliband pledged at the launch of his party's manifesto on Friday to stop what he characterised as the creeping privatisation of the NHS.

- Smaller parties muscle in -

Both parties are fighting the election on the areas on which they have always been strong, a cautious strategy informed in part by the new political landscape.

Where once the Tories and Labour could count on only having each other to fight, their shrinking share of the vote has given prominence - and power - to smaller parties.

Cameron formed a coalition after failing to win a majority, but the Lib Dems have been bruised by their time in government and are braced to lose half its 57 seats.

Meanwhile the Scottish National Party (SNP) has emerged as a new force in national politics, despite losing last year's independence referendum, and looks set to take most of the seats north of the border.

Most of these are at Labour's expense, raising the prospect of an alliance with the SNP.

The Tories have their own challenge from the anti-EU, anti-immigration UK Independence Party (UKIP), which is expected to take a handful of their seats and leech away support in others.