The claim was dismissed by Labour as "nonsense" - and by the Liberal Democrats as a "joke".
" This is the usual Tory nonsense "
The exchanges came as French Prime Minister Lionel Jospin rejected German proposals for a more federal Europe - but called for the harmonisation of business taxes.
The Tories, at their Monday news conference, also accused Labour of planning to rig any referendum on the euro.
They demanded a guarantee that any vote on the issue would be "even-handed".
Shadow chancellor Michael Portillo told a Tory news conference: "People should be very wary of being offered a referendum by the Labour government because it is perfectly clear that the Labour government intends to rig the referendum."
He said the government would "fix" the funding of the vote - and be able to choose the timing and the question to be asked.
Mr Portillo also pressed Labour to "come clean" about how much it would cost to join the single currency.
Citing a study from a City accountancy firm, he said the final bill could be £36bn - £1,500 per household, or the equivalent of 36 new Millennium Domes.
But later, shadow foreign secretary Francis Maude said the £36bn figure might not be correct.
He said: "It may not be the right figure.
"If the government has a better figure, let it share it with the public."
Prime Minister Tony Blair ridiculed the Conservative claims.
He described them as "the usual Tory nonsense".
And Chancellor Gordon Brown accused the Conservatives of a "desperate attempt" to divert attention from their own plans for £20bn in cuts.
"The whole emphasis of the Conservative campaign is now to run a one issue obsession about the euro," he said.
He insisted that the government's five economic tests on joining the euro would have to be assessed before any referendum question could be set.
Lib Dem leader Charles Kennedy joined in attacks on the Tory claims - saying they were a joke.
"I think the country frankly will have a bit of a belly laugh at the party which took us into the exchange rate mechanism at the wrong rate, then almost had to sell the crown jewels to try and maintain sterling's position in it before its ignominious departure," he said.
"I don't think that people have such short memories that the Conservatives are liable to hoodwink anyone into believing the figures they are coming forward with at the moment."
Meanwhile, French Prime Minister Lionel Jospin rejected German proposals for a more federal Europe and insisted on the central role of nation states as decision-makers in the 15-member bloc.
But at the same time, Mr Jospin, in a keynote speech, called for the harmonisation of business taxes across Europe.
This was seized on by the Tories.
William Hague said it was evidence of constant pressure to give away control of taxation and other essential national powers.
But Tony Blair denied this, saying Labour had always opposed harmonised taxes and always "won through".
He was barracked by Keep the Pound and fuel duty protesters as he visited a fete in his Sedgefield constituency where he took part in a football kick-about with youngsters.
While there the prime minister said Mr Hague was "absolutely obsessed" with the euro.
"But I will tell you what I'm obsessed about. I'm obsessed about keeping our economy strong, our mortgages low, the number of jobs high, and then putting schools and hospitals first so that we get in the investment we need."
In other campaigning on Monday, Labour announced plans for a new National Lottery fund to target £150m at some of the country's most deprived areas.
The Liberal Democrats proposed ambitious targets for household recycling and cuts in global warming emissions as part of a package of green election commitments.