Stephen Byers has revealed that a vote on euro membership is planned by the end of next year, forcing Downing Street to reject the embattled minister's claim.

The transport secretary was named by Labour MP Ian Davidson, leader of the Labour Against the Euro campaign group, as the source of an off-the-record briefing to journalists on Thursday.

During the briefing the unnamed "senior minister" said that a Referendum Bill would not be included in the Queen's Speech, but that journalists would then be briefed "the same day" that the bill would be forthcoming.

Downing Street said the suggestion that there were any plans to introduce legislation were "wrong".

"It is wrong to suggest there are any plans for legislation unless and until the five tests are met," said Tony Blair's official spokesman.

But also speaking to journalists on Thursday, Commons leader Robin Cook confirmed that a euro referendum bill could be introduced without being included in the next Queen's Speech.

"You certainly can bring in any legislation without necessarily mentioning it in the Queen's Speech. What we want the Queen's Speech to do is focus on some of the key issues, not necessarily focus on other pieces of legislation," he said.

But Cook also emphasised that there would be no bill until the Treasury's five economic tests for joining had been met.

The shadow chief secretary to the Treasury, John Bercow, told the BBC that discussing the issue through off-the-record briefings was not the way to proceed.

"This subject should not be dealt with, as it apparently has been today, by an off-the-record briefing to journalists by Stephen Byers, who is the most discredited member of this Labour government," he said.

Liberal Democrat Treasury spokesman Matthew Taylor said: "The government's position is degenerating into a cloak and dagger farce. It is increasingly apparent that the prime minister has decided on a referendum but won't admit it."

The revelations seem certain to anger Downing street and further damage the reputation of Stephen Byers.

They are set to engulf Labour in a new "spin" row and distract attention from Tony Blair's arguments in favour of joining.

In an interview broadcast on Wednesday the prime minister had given his clearest signal yet that he is prepared to go down in history as the man who scrapped the pound.

Using a second BBC Newsnight interview to make an unusually forthright declaration of support for the European single currency, Tony Blair said it would be "crazy" to stand aside from Europe.

Asked whether the government was close to a decision on Gordon Brown's much-vaunted five economic tests, Blair replied: "Yes, it is."

"Once the tests are passed we believe it is in the interests of your economy, so we believe you go in."