Many see Blair's statement as an attempt to scotch Tory calls

The Tories were set to make a referendum call a central part of their platform for the 10 June polls.

But they insist there are still other key issues for their drive for votes.

Labour and the Liberal Democrats will use the campaign to champion the EU's benefits. But Eurosceptics say their own poll prospects have been boosted.

Central issue?

The prime minister has insisted his referendum decision is aimed at confronting the "myths" being spread about Europe.

But some pro-European campaigners are accusing Tony Blair of putting short-term fears for votes in June ahead of a genuine strategy for Britain's role in the EU.

Liberal Democrat chief executive Lord Rennard instead welcomed the referendum decision, saying it was right voters had the final say on the constitution.

What Tony Blair has done is put into the limelight the question about the European Union, which is exactly what we want Mike Nattraff UKIP

"Clearly this removes the central plank of the Conservative campaign, not only for the European elections but possibly for the general election ahead," he told BBC News Online.

The party campaigns' chief said the Lib Dems hoped to benefit generally from its stance against the Iraq war, against university top-up fees and on replacing council tax.

"Specifically for the European elections, I would say the case for some of the benefits of the EU in terms of peace and security, prosperity and action on the environment would be strong Lib Dem themes," he said.

Separate vote

But Lord Rennard did not think the June polls would be a "pseudo-referendum".

"I think people understand that if Britain is to sign up to the constitution, there will be a separate choice on a different day about that."

A Conservative spokesman said the party's election campaign had never been entirely focused on the call for a referendum.

He said the Tories would continue to argue there was no reason why the vote could not be held soon after the constitution was agreed, rather than waiting for Parliament to scrutinise it.

Regional issues

There would be debate too on whether or not there should be a constitution.

"It is clear from the polls how important people see not just having the referendum but also the issue itself," said the spokesman.

Tory candidates would also be focusing on European issues touching their own regions, he added.

The key problem with the European elections is to prove the relevance of Europe Ben Page Mori

Labour is thought likely to use the campaign to highlight the benefits of Britain playing a major role in Europe, a central point of Mr Blair's Commons statement on Tuesday.

And with negotiations on the constitution due to close a week after the elections, ministers will emphasise their determination to keep British vetoes on key policy areas.

But Mike Nattraff, deputy leader of the UK Independence Party (UKIP), said public focus on the referendum had bolstered its campaign posters opposing the EU constitution.

"This is encouraging people," he told BBC News Online. "What Tony Blair has done is put into the limelight the question about the European Union, which is exactly what we want."

An advertisement in The Sun newspaper for UKIP, which wants Britain to withdraw from the EU, had generated 6,500 calls in support, said Mr Nattraff.

The campaign should be about "who governs Britain", he added.

Voter interest?

Ben Page, director of the MORI Social Research Institute, said only "terrifying" voters about the constitution would make it a major issue which would encourage people to vote.

"I don't think the fact there is going to be a referendum is necessarily going to make much difference to the European elections," he told BBC News Online.

While there was often 70-80% of people calling for referendum on issues, when the votes came turnout was much lower.

"The key problem with the European elections is to prove the relevance of Europe," he added.