Business leaders lined up to back the plans for a new campaign this weekend.
Gavin Patterson, chief executive of British Telecom, which employs 85,000, said the EU was a good thing for business in spite of its problems.
He said: "Like most business leaders, I think we will be campaigning on the basis that the EU, for all its idiosyncrasies, is a positive thing for business. We do a lot of business across Europe. We have significant businesses in Italy, Germany, France, the Netherlands, Spain, Switzerland and it will create uncertainty. Companies generally will look to move investment away from the UK."
Sir Roger Carr, chairman of the defence giant BAE Systems, said: "With a Conservative victory, we have the certainty of both a pro-business Government and a referendum on Europe.
"British business must now stand up and speak up for membership of an improved Europe if we are to capitalise on the opportunities that the future now offers."
There was double relief in the City that, as well as a Conservative victory, the election delivered the stability of a single party majority Government. The FTSE 250 hit a record on Saturday as investors gained confidence.
Michael Sherwood, joint chief executive of Goldman Sachs International, said: "I am very encouraged by the result and happy David Cameron will be able to present the European debate to the country as a majority Government. I hope that after extensive discussion, the case for remaining in Europe will resonate with the British public."
More vocal support from business is a boost to the Prime Minister, who wants to renegotiate the terms of Britain's EU membership but retain a seat at the table and the benefits of free trade within the bloc.
Cameron made the promise of a referendum during the last Parliament under pressure from Eurosceptic backbenchers who believed they were being outflanked to the right on the EU by the UK Independence Party.
Ukip's failure to take any seats from the Conservatives could also strengthen his hand in the debate.
Sir Martin Sorrell, chief executive of advertising group WPP, said the collapse of the Liberal Democrats could loosen ties as EU support was a main policy pledge.
On the other hand, he said, the SNP now dominates Scottish representation at Westminster and is in favour of membership.
Telegraph Group Ltd