The U. S. government has warned Britain against quitting the European Union, threatening to exclude the UK from a US - EU trade and investment partnership potentially worth hundreds of billions of pounds a year, media report said.The Obama government also warned that it was very unlikely that Washington would clinch a separate deal with Britain. The warning comes in the wake of Prime Minister David Cameron's visit to Washington, which was primarily intended as a joint promotion of the Transatlantic Trade and Investment Partnership (TTIP) with Barak Obama. Cameron said the deal could bring £10bn a year to the UK alone. However, it was overshadowed by a cabinet rebellion back in London, where cabinet ministers threatened to support a UK exit from the EU. The threat raised doubts in Washington on whether Britain would still be part of the deal once it had been negotiated. More immediately, Obama government officials were concerned that the uncertainty over Britain's future would further complicate what is already a hard sell in Congress, threatening a central pledge in the president's State of the Union address in February. "Having Britain in the EU … is going to strengthen the possibility that we succeed in a very difficult negotiation, as it involves so many different interests and having Britain as a key player and pushing for this will be important," a senior US official said. "We have expressed our views of Britain's role in the EU and they haven't changed. TTIP negotiations underscore why we think it's important that it continues." If it is successful the agreement could be the biggest trade and investment deal in history, encompassing half the world's GDP and a third of its trade.