Prime Minister David Cameron's Conservative Party is three percentage points ahead of the Labour Party, an opinion poll showed on Monday, three months before a May 7 national election.

A three-point lead, broadly within the margin of error of most opinion polls, underlines how close the contest, billed as the most unpredictable in modern British history, really is.

Most recent polls have put Labour narrowly in the lead or neck and neck with the Conservatives.

Monday's poll, funded by Michael Ashcroft, a former deputy chairman of the Conservatives, put Cameron's party on 34 per cent, Labour on 31 per cent, the Lib Dems on 9 per cent, the Greens on 6 per cent and the UK Independence Party (UKIP) on 14 per cent.

Analysts believe Cameron's party would need to open up a lead of up to seven percentage points to have a realistic chance of winning the election outright under Britain's winner takes all voting system. A quirk of the current system means Labour could win a majority with a smaller lead of around five points.

The poll was conducted on February 6-8 and interviewed 1,003 adults by phone. Results were weighted. Ashcroft polls, despite the political past of their funder, are widely accepted to be authoritative and impartial.