The Tories demanded a change to the small print of the draft Bill on party political funding published by Jack Straw, the Home Secretary, on Tuesday.
Although Mr Straw announced that the budgets of the pro and anti euro campaigns would both be limited to pounds 5m, he also proposed that each political party with two or more MPs should be allowed to spend the same amount. The Tories say this would not be the "level playing field" promised by the Government, because more parties support the single currency than oppose it.
Labour, the Liberal Democrats, the Scottish National Party and Plaid Cymru, who would be in the "yes" camp, would be able to spend pounds 20m between them. On the "no" side, the Tories would be able to spend pounds 5m.
The "yes" campaign would have a total of pounds 25m, the "no" camp only pounds 10m.
Mr Straw has rejected the advice of the Neill Committee on Standards in Public Life, on whose inquiry into party funding his draft Bill is based. It recommended against limits on party spending during referendum campaigns.
"Under these rules, a Labour-led coalition to scrap the pound would be able to outspend the Conservatives by a factor of four," said Ann Widdecombe, the shadow Home Secretary. "Tony Blair ... is rigging the rules on funding in favour of his campaign to scrap the pound."
The Tories pledged to amend the "unfair and undemocratic" rules during the passage of the Bill through Parliament in the session starting in November.
While the smaller parties are unlikely to have pounds 5m to spend, the Tories believe they could provide channels for cash from pro-European business groups.
The Government will allow parties without two MPs and other organisations and people to spend pounds 500,000 each during a referendum campaign.
The Neill committee argued against limits because of the transient nature of campaign groups compared with the longevity of political parties.
But the Home Office said: "The Government is not persuaded that limits on spending, provided that they do not attempt to do too much, need be ineffective or would raise fresh issues of principle."Reuse content