The European Commission is set to consider plans that could see a reduction in Britain's £2 billion EU budget rebate.

Under the proposals, Britain would become the single biggest contributor to the Brussels budget, paying half as much again as France.

Michaele Schreyer, the EU budget commissioner, aims to replace the UK's rebate with a general system of refunds for all big contributors.

The proposals could increase the pressure on Tony Blair ahead of any referendum on the EU constitution.

While the constitution maintains the national veto on budgetary issues, there will be sustained pressure from other countries for Britain to give ground.

The rebate was famously won by then prime minister Margaret Thatcher in 1984, who demanded: "I want my money back."

And the government has pledged to defend the rebate, with officials describing it as "not negotiable".

"The rebate is fully justified and is not on the table," said a British government spokesman.

"The reason the rebate exists is because of the massive distortion of EU spending, especially on farm subsidies, which hugely benefit France as well as other countries but give little to the UK."

Number 10 added that "the position hasn't changed".

"The distortions in expenditure still remain," the official spokesman said.

But Downing Street stressed that "the Commission hasn't yet made its proposals".

Meanwhile, there have been further tensions between the Commission and the UK government.

Gordon Brown has been given two months to drop "severe and intrusive" sanctions against cross-Channel shoppers who fall foul of government limits on cheap alcohol and cigarettes.

The Commission threatened to refer Britain to the European Court of Justice unless it abandons the "disproportionate" methods used by Customs and Excise to stop what they claim is tax evasion.