The Conservatives have called on the government to hold an immediate referendum on the single currency.

In a speech on Tuesday afternoon, Iain Duncan Smith said that his party would never join the euro and called for an immediate poll to settle the issue once and for all.

He urged the government to "call a referendum now and find out exactly how the British people feel about the euro".

Breaking his self-imposed vow of silence on Europe, the Conservative leader described the government's five tests as "a sham" and "a smokescreen".

"The Conservatives would not take Britain into the euro," Duncan Smith vowed, saying that most British people agreed with his views.

Warning the tests had not been met, he said that interest rates in the eurozone would not always meet British needs.

The Tory leader also said that leaving open the option of a referendum later in this parliament could cause damaging economic instability.

"The prime minister is taking a gamble in which the only loser will be Britain. so if Tony Blair believes Britain should join the euro. then he should say so and get on with calling a referendum to find out exactly what the British people think."

He said the reform and renewal of public services should instead be the priority for ministers.

The speech comes ahead of Gordon Brown's announcement on the outcome of his five tests on euro membership.

The chancellor is expected to say that Britain has not yet met the criteria needed to join - but will leave the door open for an examination of the issue in the second half of the parliament.

Duncan Smith's decision to speak out on the euro is being seen as a sense of renewed confidence that the Conservatives can mount a united attack on the question of Europe.

The Conservative leader used his speech to launch a fierce attack on Labour's record in government, saying that they had failed to improve public services despite taxing and spending more.

And now, emboldened by last week's local election gains, the Tory leader hopes to capitalise on the public's opposition to euro membership.

But pressure for an early poll grew after Robin Cook called for the government to set a firm date for euro membership.

Warning that delay is not an option, the former foreign secretary said that Britain would be sidelined in Europe unless it starts the process of joining the single currency.

"No member of the eurozone is going to listen to our chancellor if the government has just ruled out Britain becoming a member," he said.

"A failure now to commit ourselves to membership would lose Britain its unique status as a 'pre-in' and reduce us to a definite out."

Cook also warned that ministers should be bold in explaining the advantages of the single currency to voters.

"The opinion polls confirm the predictable public rift with France and Germany has made it more challenging to ask Britain to vote for greater integration with those partners. We have pursued a US political priority in Iraq at a cost to British interests in Europe," he said.