"But in return, Europe would have to give up existing protections on its agriculture, film industry and public services."
The Obama administration has warned British officials that if the UK leaves Europe it will exclude itself from a US-EU trade and investment partnership potentially worth hundreds of billions of pounds a year, and that it was very unlikely that Washington would make a separate deal with Britain.
The warning comes in the wake of David Cameron's visit to Washington, which was primarily intended as a joint promotion of the Transatlantic Trade and Investment Partnership (TTIP) with Barack Obama, which the prime minister said could bring £10bn a year to the UK alone, but which was overshadowed by a cabinet rebellion back in London.
The threat by Cameron's ministers to back a UK exit in a referendum on the EU raised doubts in Washington on whether Britain would still be part of the deal once it had been negotiated.US officials say that the White House is particularly perplexed because Britain played a key role in persuading Obama to stake significant political capital on the ambitious transatlantic partnership.
If it is successful it could be the biggest trade and investment deal in history, encompassing half the world's GDP and a third of its trade.
Both European and American officials involved in preparatory talks on the partnership said that it was also intended as a bulwark against the economic challenge of China, aimed at forming a bloc powerful enough to lay down the rules of international trade and investment.
The TTIP will aim to remove the relatively low tariffs of about 3% to 5% between the US and Europe, but its greatest impact will be felt in promoting investment in both directions largely by the convergence of regulations on either side of the Atlantic.
One of the greatest potential advantages for the EU would be the opening up of tenders at the US state level to European suppliers. But in return, Europe would have to give up existing protections on its agriculture, film industry and public services.