David Cameron came under pressure from eurosceptics in the City 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."

They continued: "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 on Friday 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?