The risks of leaving the European Union "far outweigh the opportunities", according to the head of an influential foreign affairs think tank.
Robin Niblett, director of Chatham House, said Brexit would lose the country influence and "weaken its sovereign power" outside the EU.
He acknowledged that membership of the bloc meant giving up control over the flow of workers from the EU, but stressed that migration had benefited the economy and free movement was also a valuable right for Britons.
The Chatham House paper argues that as a member of the European Union, Britain retains control of 98% of government spending and sets its own policies on almost every issue of serious concern to British voters.
Dr Niblett said "The debate in Britain over sovereignty is in a sorry state - absolute sovereignty is worthless if it reduces the prosperity and security of British citizens.
"The important thing is the government's ability to secure outcomes in the interests of the British people - and that is unquestionably enhanced by membership of the European Union."
His paper acknowledged that migration of workers from the EU has caused "great public concern and is a principal driver of support for the campaign to leave".
"There is no escaping this trade-off, as it is a key requirement of EU membership. However, free movement of labour is a benefit for the UK economy in the aggregate, and a valuable right for British citizens who wish to take up employment in other EU countries."
The paper concluded: "The fundamental question before the British electorate, therefore, is not whether it is time for Britain 'to take back control' from the EU.
"This is a worthless proposition if it does not address Britain's actual ability to ensure the prosperity and security of its citizens.
"The question should be whether Britons are content with the trade-off that accompanies EU membership: that is, that Britain can best enhance its security and its economic prospects if it accepts being part of a single market for workers as well as for goods, services and capital.
"In my opinion, the choice is clear. For Britain, the opportunities that will come from remaining in the EU far outweigh the risks, and the risks of leaving far outweigh the opportunities. Britain's effective sovereignty will be strengthened, therefore, if it remains a member of the EU."
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