The message is at odds with pleas from key City figures, including Bank of England governor Mark Carney, to cast the in/out vote as quickly as possible to end uncertainty affecting the financial sector and UK companies.
But Spencer, head of interdealer broking giant Icap, said the issue should not be rushed.
The former Tory party treasurer, who has donated over £4m to the Conservative party, said the UK requires a referendum “as the issue needs to be put to bed”. But he added: “We need to give a proper amount of time for the PM to negotiate and achieve substantive change.”
Spencer’s comments come amid reports that the Prime Minister wants to fast-track negotiations on EU reform. It is widely expected that an EU referendum bill will be included in next week’s Queen Speech, and reports yesterday indicated that the Prime Minister will push for an in/out vote to be held next year.
Cameron had previously promised a referendum by the end of 2017.
His message also coincides with a push from Confederation of British Industry (CBI) president Sir Mike Rake for business leaders to campaign in favour of staying in the EU.
“Business has increasingly spoken out on this crucial issue and the time has come to turn up the volume,” Rake will say at the business group’s annual dinner. Chancellor George Osborne, who will lead the PM’s renegotiation efforts in Brussels, is expected to be among the audience.
The threat of a British exit from the EU – known as a Brexit – is increasingly dividing the City.
While JCB chief executive Graeme MacDonald has said he would back Britain leaving the currency bloc, Vodafone chief executive Vittorio Colao told City A.M. “it is in the interest of our shareholders and customers that Britain stays in the EU.”
Open Europe, a think-tank that campaigns for a reformed EU, yesterday called for a cautious approach: “The risk is that an early referendum – without a full reform process – fails to settle the issue, leading to greater uncertainty in the medium term.”
But Anthony Browne of the British Bankers’ Association (BBA), urged a swift decision because uncertainty was discouraging investment in British companies.
“One international bank has made a board-level decision to move operations out of London if Britain leaves the EU,” he told City A.M.
Deutsche Bank, which employs 9,000 in the UK, announced it had begun making contingency plans on whether to quit the UK in case of a Brexit, and Browne said many others are also considering their options.