The UK Government must stop sounding like Margaret Thatcher in its negotiations in Brussels, European Commissioner Peter Mandelson has said.
Mr Mandelson is worried about the burden on poorer nations
In a speech in London, Mr Mandelson said the UK had to change both tone and substance to win backing in Europe.
The UK must be ready to look at reforming its rebate as part of a deeper rethink of EU spending, he said.
"It is surely wrong to ask the poorer new accession states to pay for any part of the rebate," he said.
Mr Mandelson's intervention came as Tony Blair continued a round of shuttle diplomacy ahead of this week's European summit.
The prime minister said he would be "diplomatic but firm" in insisting he would not compromise on the rebate unless there was a full review of European farms subsidies.
French European Affairs minister Catherine Colonna says the UK's defence of its £3bn rebate deal "defies logic".
In a Fabian lecture in east London, Mr Mandelson said: "Refusal to talk about much needed budget reform is part of the old conservatism in Europe which the Barroso Commission is determined to change.
"But Britain should be careful not to play into the hands of this conservatism.
"Ministers must be consistent and courageous in their reformism, and be prepared, in the context of a deeper re-think about the EU's budget, to look at reforming Britain's rebate."
It was Mrs Thatcher who negotiated the rebate in 1984 and Mr Mandelson was worried ministers were following her style.
He said: "There is a paradox that in Britain, New Labour have been strong advocates of a modern social democratic mix of market flexibility with massive public investment and the first successful attack on poverty in a generation.
"In Brussels, Britain has sounded neo-Thatcherite as though nothing has changed from the 1980s.
"Both tone and substance need now to change if the British Government is to command attention and win the backing it seeks on the continent.
"A greater effort must be made to get this right during the UK Presidency [of the EU] which starts in a few weeks' time."
Asked about the comments, a Downing Street spokeswoman said only that Mr Mandelson was a European commissioner and the prime minister had made his views clear.