Britain's opposition Labour party piled pressure on Prime Minister David Cameron on Monday by pushing for an even tougher line on the European Union budget than that proposed by his ruling Conservative Party.
Cameron is promising to take a tough line at what are shaping up to be fraught talks next month to agree the EU's next seven-year budget in the face of growing anti-EU sentiment in Britain.
Last week he reiterated a threat to veto any budget deal seen as detrimental to British taxpayers, and demanded a real-terms freeze in EU spending given the financial constraints and budget cuts faced by many European governments.
Now left-leaning Labour has raised the stakes by demanding a cut in real terms.
Many Britons regard the EU as an ineffectual and spendthrift source of bureaucracy and Britain's ties with the 27-member bloc are likely to be a key theme in a national election set for 2015.
The centre-right Conservative vote is already under threat from the UK Independence Party, which has surged in popularity in recent months on a pledge to withdraw Britain from the EU.
"Labour will argue against the proposed increase in EU spending and instead support a real-terms cut in the budget," Labour finance spokesman Ed Balls and foreign affairs spokesman Douglas Alexander said in a joint opinion piece in the right-leaning Times newspaper.
Labour has moved clear of the Conservatives in opinion polls since Cameron's party came to power at the head of a coalition government in 2010.
David Lidington, a Conservative and the government's minister for Europe, said Labour had "zero credibility on standing up for Britain in Europe".
"They waved through above inflation increases for both of the multi-year budgets they approved ... We won't take any lessons from them about budget negotiation," Lidington said in a statement.
Cameron wants to remain within the EU given that it accounts for about half of British trade. But he has pledged to negotiate a new settlement with Brussels then seek the public's "fresh consent" for the deal, giving no timeline.
Making negotiations more difficult for Cameron are signs of growing irritation in Europe over what some EU leaders regard as British isolationism and opportunist demands at a time when governments are trying to fix the euro zone debt crisis.
In December, Cameron vetoed a European economic and fiscal pact to help the EU's euro zone countries recover from sovereign debt crises that had cast doubt on the 17-member single currency.
"As a result of David Cameron's behaviour, those we used to call friends now ridicule the prime minister in meetings, shut him out of negotiations and bad-mouth him to the press," the Labour spokesmen said in their article.
Prominent Conservative commentator and activist Tim Montgomerie wrote on his ConservativeHome website that Labour's move was "clever politics" because it created a "headache" for Cameron by appealing to euro sceptic newspapers and voters.
(Additional reporting by Matt Falloon; editing by Robert Woodward)