The chief executive of money broker ICAP has said the UK "can thrive" outside the European Union.
Speaking to the BBC, Michael Spencer said: "Yes we can thrive. The UK is the fifth largest economy in the world, it's absolutely an economic leader in many aspects."
However, Mr Spencer said there were "pros" to Britain staying in Europe.
Prime Minister David Cameron has promised an in-out referendum on the UK's EU membership by the end of 2017.
Mr Cameron has said he will campaign for Britain to remain in the EU if he gets the reforms he wants.
Mr Spencer - who was treasurer for the Conservative party up until 2010 and remains a donor - said he had not yet made his mind up on how to vote.
The decision, he said, would depend on the terms offered by the EU for Britain to stay.
"Europe should be prepared to make proper concessions to us," he said.
In or out?
Many business leaders have said they are in favour of staying within Europe.
In November, Vodafone chief executive Vittorio Colao told the BBC that it was important for its business that Britain remained part of the EU, saying it's "good for our customers, it's good for our shareholders, and it's good for the company itself".
In September, a survey of 2,000 businesses by the British Chambers of Commerce found that 63% would vote to stay in the EU, with 27% voting "no".
However, half said their final view depended on the outcome of Mr Cameron's negotiations over EU reform.
Earlier this year, the chairman of construction equipment firm JCB said the UK should not fear an exit from the European Union.
"We are the fifth or sixth largest economy in the world. We could exist on our own - peacefully and sensibly," Lord Bamford told the BBC.