LONDON (AFP) - Prime Minister David Cameron's pledge to hold a referendum on whether Britain should remain in the EU appears to have boosted his party's ratings at home, opinion polls published on Sunday suggest.

Support for Cameron's Conservative Party jumped five points from the previous month in a poll conducted after he vowed Wednesday to hold an "in-or-out" vote on Europe if the party wins the next general election.

This was despite 38 percent of those surveyed saying they believed that leaving the EU would lead to a loss of jobs and trade and harm the economy.

The poll conducted by ComRes for the Independent on Sunday and Sunday Mirror newspapers put the Conservatives on 33 percent, up from 28 percent in December. That halved the lead of the main opposition Labour party, whose ratings stood unchanged at 39 percent.

Separate polls conducted for the Mail on Sunday and Sunday Express showed a similar rise in support for the Tories following the speech.

Cameron's centre-right party appeared to have won over supporters of the resurgent anti-Europe UK Independence Party, which lost four points to 10 percent.

However support for the Liberal Democrats, Britain's junior coalition partners, swelled from nine percent to 11 percent.

Lib Dem leader Nick Clegg, the deputy prime minister, criticised Cameron's referendum pledge, arguing that it would cause "years of uncertainty" and damage the economy.

Labour has also rejected the idea of an in-out vote.

In his long-awaited speech in London Cameron promised to hold a referendum by the end of 2017 if his Conservatives got into power at the next general election in 2015.

But he said he wanted to renegotiate the terms of Britain's troubled EU membership first and repatriate some powers before putting the new agreement to a vote.

Former prime minister Tony Blair, in an article in the Mail on Sunday, said a British exit from the EU would be an "extraordinary denial of its own interests".

He called on the country's pro-Europe camp to start making a case for a "yes" vote, saying: "I believe there is a sensible, solid majority in the UK for us to stay in Europe. It is time to start mobilising it."

Meanwhile the Conservative Mayor of London Boris Johnson said in an interview with the Sunday Telegraph that he would consider voting to leave the EU if Britain's demands were not met.

"I will vote yes if we get the right deal, but obviously logically I can't rule out voting no," he told the right-leaning newspaper.