A £6m campaign to spread information about the EU Constitution could be illegal, a Conservative MEP has said.
Most EU citizens have not read the European Constitution
EU officials claim the drive will only inform and not promote a "yes" vote in referendums on the Constitution.
But Den Dover, who led the fight against the plan approved by MEPs on Tuesday, said it was part of an agenda to win support for the treaty.
Downing Street says it has already rejected any cash which could be seen as "pro-constitution propaganda".
A government spokesman said: "Both the yes and no campaigns will be spending a lot of money on this issue, and we do not want to add to the pot money which comes from the EU budget - effectively British taxpayers' money."
MEPs had ear-marked a total of £80,000 for the UK.
Tory MEP Den Dover, who led opposition to the campaign in the European Parliament, told BBC Radio 4's Today Programme: "It is unacceptable. I think it is illegal quite frankly.
"We were told informally that this would not be for or against, but there is a huge agenda within the European Parliament to get people to vote for the constitution."
UKIP MEP Nigel Farage said the campaign was a "monstrous abuse of taxpayers' money" and an "attempt to bully the electorate into voting for this totally unacceptable constitution".
But a spokesman for the European Parliament said the campaign was concerned with spreading information on the "main aspects of the constitution".
Den Dover MEP led the opposition to the information campaign
He also pointed out that the European Parliament had voted in favour of the EU constitution.
The campaign would run the 25 countries of the enlarged EU not just those holding referendums on the treaty, he added.
But Mr Farage said the plans for the campaign approved by MEPs read: "We will explain to the public how the constitution will benefit them in their every day lives."
According to polls, only 11% of EU residents have read the constitution, which changes the way the union works.
The constitution must be ratified by all 25 member states by November 2006 if it is to come into effect.
It is designed to streamline the way decisions are taken in an EU that now includes 25 countries, rather than the original six.
It also gives the enlarged EU its own president and foreign minister.
But critics argue the reforms would turn the EU into a "superstate".
Spanish voters voted in favour of the constitution in February. No date for a UK referendum has yet been set, but it is not expected before 2006.