David Cameron is under pressure on Europe from Tory critics and Labour ahead of a crunch summit about the EU budget this week.
The Prime Minister is preparing for crucial talks on the group's budget for 2014-2020, where he has vowed to veto anything but a real-terms freeze.
Ahead of the meeting, Mayor of London Boris Johnson urged Mr Cameron to emulate Margaret Thatcher and stand firm against other European leaders.
And leading right-winger David Davis warned in a speech about Britain's relationship with Europe that it is "time to decide".
Meanwhile, Labour leader Ed Miliband called for sweeping reforms to ensure Britain is not sidelined in Brussels as he recommitted himself to the EU.
The speeches came amid reports that EU officials are working on a long-term budget that could be agreed annually and would not need the UK's approval.
Mr Miliband warned that the UK will "sleepwalk" into an exit that could wreck the British economy if pro-Europeans fail to speak up and make the case for change.
He told business leaders at the CBI that eurosceptic Tories have forced Mr Cameron into "negotiations that will not deliver" for the repatriation of swathes of powers.
Mr Miliband claims the Prime Minister should be focusing on building alliances and ensuring Britain will not suffer once eurozone countries have deepened their links.
In a bid to win over the business vote, he said it is right to press for reform in some areas while insisting that EU membership is in the national interest.
"Those of us, like me, who passionately believe that Britain is stronger in the European Union cannot be silent in a situation like this," he said.
"I will not allow our country to sleepwalk toward exit because it would be a betrayal of our national interest."
The latest opinion poll, a survey by ComRes this weekend, shows 43% of the public want Britain to pull out of the EU regardless of whether powers can be repatriated or not.
Mr Cameron has claimed the eurozone crisis is a chance to renegotiate Britain's relationship with Brussels and promised to put any new deal to the public in a referendum.
He is battling to keep his party in line over the issue, which recently saw rebels join with Labour to defeat the Government by voting for a real-terms cut in the EU budget.
The Prime Minister himself said he would prefer a cut and promised to use his veto against anything but a freeze but this was not enough to convince backbenchers.
On Monday, he insisted again that it was "not credible" to agree rises to the EU budget after insisting on austerity at home.
"I feel I have got the people of Europe on my side in arguing that we should stop endlessly picking their pockets and spending more and more money through the EU budget," he said.
Mr Johnson, writing in the Daily Telegraph, called on Mr Cameron to veto any deal that would increase spending, even beyond the deadline for an agreement next year.
Calling on him to follow the tough approach adopted by Baroness Thatcher in the 80s, he said: "It is time for David Cameron to put on that pineapple-coloured wig and powder-blue suit, whirl his handbag round his head and bring it crashing to the table with the words: No, non, nein."
The Prime Minister is expected to make a key speech on the wider Europe question before Christmas, when he is predicted to promise a referendum after the next election.
Mr Davis, a former Europe minister, wants a double referendum - one to approve a list of powers for the UK to try and seize back and then an in/out poll once that was negotiated.
He is calling for the first vote to be held within a year, before the next elections to the European Parliament in 2014.
Veteran Cabinet pro-European Ken Clarke has joined the argument, insisting it would be a "disaster" for Britain if it withdrew.
"Putting our membership of the European Union at risk is complete folly and irresponsible debate about it at the moment weakens Britain's role at a table where a lot of very important things have to be said," he said.
But even Business Secretary Vince Cable, who supports British involvement in Europe, admitted the public will have to be consulted at some point.
"Even those of us who are in favour of British participation in Europe and supportive of it have got to carry this debate to the country," he told BBC Radio 4's The World At One.
"There will have to be a public argument, there will be a referendum at some point. The issue at the moment is about timing."