EU trade commissioner Peter Mandelson has warned pro-European MPs not to get drawn into an anti-Europe campaign by supporting an EU Treaty referendum.
Mr Mandelson said those calling for a referendum often wanted the UK to withdraw from the EU.
"I'm afraid those pro-Europeans arguing for a referendum risk being drawn into supporting this agenda," he said.
On Thursday a cross-party group of MPs launched a campaign aimed at pressuring Gordon Brown into holding a referendum.
But Mr Mandelson said a return to the "poisonous" debates of the past on Europe would reduce the UK's influence.
Constitution dropped
He added: "It is not for me to express a view on the UK's domestic decision about a referendum, but I note the British government says that this is not a treaty which requires one.
"Britain is not a country governed by the use of referenda. And those who argue for one in reality all too often want Britain to withdraw.
"I am afraid those pro-Europeans arguing for a referendum risk being drawn into supporting this agenda."
"If we succeed in achieving our red lines in all the detailed negotiations, there will, in my view and in the government's view, be no need for a referendum"
The Labour government had promised a referendum on the EU Constitution - but that was dropped after being rejected by French and Dutch voters in 2005.
But the government argues the reform treaty is different to the constitution and Britain has got its way in negotiations on its "red lines" - key areas like human and social rights, foreign policy and tax and benefits.
Mr Brown has stopped short of ruling out a referendum, but the government has said the constitution has been abandoned and Britain's "red lines" are secure.
He told the BBC this week: "If we succeed in achieving our red lines in all the detailed negotiations, there will, in my view and in the government's view, be no need for a referendum."
But critics say most of the constitution remains in the treaty. Those supporting the I Want a Referendum campaign say it is a matter of trust for voters.
They say they already have the support of about 50 Labour MPs and the sympathy of up to 70 more.
They believe that if 100 Labour MPs sign a Commons motion calling for a referendum, the prime minister will be forced to hold a vote.
Former Europe minister Keith Vaz has called for a referendum - saying pro-European MPs should not be afraid of putting the argument to the British people - an argument he thinks they can win.
The treaty, which is expected to be finalised later this year, was drawn up to reform the 27-member EU, after the constitution was abandoned.
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