The Labour Party has taken the lead over Prime Minister David Cameron's Conservatives, according to four opinion polls less than a month before the May 7 election, with one survey showing a Labour lead of six percentage points.

Neither Cameron's Conservatives nor Ed Miliband's Labour Party is forecast to win an overall majority in Britain's 650-seat parliament after losing support to once fringe parties such as the Scottish National Party and the UK Independence Party.

An online Panelbase poll published Thursday put Labour up four points on 37 per cent, the Conservatives down two points on 31 per cent, UKIP down one point on 16 per cent and the Liberal Democrats up one point on eight per cent.

A different online survey by polling firm Survation and conducted for the Daily Mirror newspaper gave Labour a four percentage-point lead over the Conservatives, the biggest Labour lead in a Survation poll since February.

The Survation poll also showed Miliband had overtaken Cameron in terms of net personal approval ratings, the first time Miliband had outpolled Cameron since Survation began asking voters about personal approval ratings in January.

The poll put support for Labour on 35 per cent, up from 33 per cent since the firm's last poll on April 3, while the Conservatives dipped one percentage point to 31 per cent.

Support for the anti-EU United Kingdom Independence Party fell three percentage points to 15 per cent, while the Lib Dems were unchanged at nine per cent. The Scottish nationalists and Greens had four per cent each, according to Survation.

Opinion polls indicate the Scottish nationalists are on course to win 35-50 of the 59 Westminster seats in Scotland, up from six in 2010, giving them a potential kingmaker position in the Westminster parliament.

The stakes of the election are high: If Cameron wins, he has promised a referendum on European Union membership, while Scottish nationalists have offered to shore up a future Labour minority government in the event of a hung parliament.

The Survation poll surveyed 1,111 people on Wednesday and Thursday. The Panelbase poll surveyed 1,013 people.

A daily YouGov poll for The Sun newspaper published on Thursday put Labour on 35 per cent, unchanged from the day before, and the Conservatives on 34 per cent, up one percentage point, The Sun said.

A TNS poll put Labour up one on 33 per cent, the Conservatives down three points on 30 per cent, UKIP up three on 19 per cent and the Lib Dems unchanged on 8 per cent.

Miliband announced on Wednesday that he wanted to scrap tax rules that allow wealthy individuals to reduce the amount of tax they pay on money earned overseas.

The Survation poll found 59 per cent of respondents supported the idea.