David Cameron came under pressure from eurosceptics in the City today to offer Britain "a clear choice" over its membership of the European Union.

Challenging the claims of pro-Europeans that business wants Britain to remain in the EU, figures including banker and Conservative peer Lord Flight, venture capitalist Jon Moulton and former Barclays director Lord Vinson called for an in/out referendum.

In a letter to the Daily Telegraph, the signatories said: "Pressure is growing from all sides for a referendum on our membership of the EU, not least because of the movement towards full banking and political union within the next five years.

"The pro-European lobby, apparently speaking on behalf of British business as a whole, has argued in generalities. We believe that members of the public should be given specific figures, not platitudes, so that they can make an informed decision about whether Britain should remain in the European Union.

"The City has for 1,000 years been at the heart of the development of British wealth and democracy. We are determined that the people of Britain be given a clear choice."

Other signatories included former Liffe chief executive Daniel Hodson, insurance executive Robert Hiscox, Adam Fleming, David Reid Scott, John Robins and Stanislas Yassukovich.

Ahead of his long-awaited speech tomorrow on Britain's relationship with the EU, the Prime Minister also faced rival demands from different wings of his divided backbenches.

One group of 25 Conservative MPs - including ex-Cabinet minister Caroline Spelman, former foreign secretary Sir Malcolm Rifkind and former vice chairman of the Conservative Party Margot James - urged him to use his speech to emphasise that Britain is better off in the EU.

In a letter, they warned that an "over-emphasis" on renegotiation and the promise of a referendum could undermine the single market, which it pointed out was created by Margaret Thatcher and enlarged by John Major.

"We fear that a renegotiation which seems to favour the UK alone would force other capitals to ask why they cannot simply dispense with those parts of the single market that don't suit them, potentially endangering Margaret Thatcher's defining European legacy," the letter said.

"Senior business figures don't want the UK to play a lesser role in the EU. They fear, as we know you do, the danger to British business and jobs of the UK being on the wrong side of a tariff barrier which could fatally undermine our government's policy of rebalancing the economy so that we boost manufacturing and reduce unemployment."

Another group of MPs - John Redwood, Bill Cash and Bernard Jenkin - will seek to counter the letter by setting out the case of Tory eurosceptics with the launch this morning of a paper called The EU Single Market - Is It Worth It?

A source said it would be a "brilliant riposte" to the pro-single market letter.

Labour leader Ed Miliband warned that Mr Cameron was about to take Britain "to the edge of an economic cliff" by creating uncertainty for business with his speech in the Netherlands.

He told the Financial Times: "If you're an investor looking at where to put money in the next x years, you'll be told by the Prime Minister on Friday, 'don't put your money in Britain because we might be out of the European Union'."

He added that Mr Cameron "should be listening to the CBI and not (UK Independence Party leader) Nigel Farage".

The Prime Minister yesterday said voters would have a choice between taking powers back for Britain with the Tories or handing power over to Brussels with Labour.

"What business wants in Europe is what I want in Europe: for us to be part of Europe but a more flexible Europe, a more competitive Europe, a Europe that can take on the challenge of the global race and the rise of nations in the south and the east," he told the Commons.

Countries throughout Europe were considering how they can pursue national priorities in forthcoming treaty changes, he said, adding: "Political parties in this country face a choice: do we look at these changes and see what we can do to maximise Britain's national interest, and do we consult the public about that, or do we sit back, do nothing and tell the public to go hang?

"I know where I stand, I know where this party stands, and that's in the national interest."

He accused Labour of not wanting to give the British people a say on Europe, and said the 2015 election would present voters with a very simple choice: "If you want to take power back for Britain you vote Conservative, if you want to give power to Brussels you vote Labour."

Another group of Conservative backbenchers yesterday released a blueprint for renegotiated membership, which they said could deliver "a new and sustainable position for the UK within the EU".

The Fresh Start Project's Manifesto for Change set out demands for five significant revisions to EU treaties, to repatriate powers on social and employment law, deliver an "emergency brake" on financial services issues, allow the UK to opt out of policing and crime measures, provide legal safeguards for the single market and abolish the European Parliament's second home in Strasbourg.

The proposals were warmly received by Foreign Secretary William Hague, who said in a foreword to the document: "Many of the proposals are already Government policy, some could well become future Government or Conservative Party policy and some may require further thought."

Fresh Start supporter and former minister Tim Loughton said that Mr Cameron should focus his energies on a "robust and achievable negotiation of our terms of membership" of the EU.

Former defence secretary Liam Fox urged the Prime Minister to use tomorrow's speech to make an "unambiguous declaration" that Britain will not continue down the road of "ever closer union" for the EU, and wants to negotiate a return to a Common Market relationship.

Following that renegotiation, British voters should be given a "clear in/out choice" in a referendum, he said.

Writing on the ConservativeHome website, Dr Fox said: "Ending the concept of 'ever closer union' by negotiating a new agreement - on the basis is that it will settle the European question in Britain for a generation with a clear in/out referendum choice for the British people based on renegotiation - is what many of us hope we will hear from the Prime Minister tomorrow.

"I believe it would forge an even greater consensus in our party and in our country, where for the first time official policy on Europe will actually mirror what the people of Britain really think.

"Clarity, courage and conviction will be needed and the road will not be easy, but the rewards could be truly historic."