The US President said a free trade arrangement would not happen any time soon in the event of a Brexit.

Speaking alongside Prime Minister David Cameron at the Foreign Office, he said: "I think it's fair to say that maybe some point down the line there might be a UK-US trade agreement.

"But it's not going to happen any time soon because our focus is in negotiating with a big bloc, the European Union, to get a trade agreement done. The UK is going to be in the back of the queue."

Trying to do "piecemeal trade agreements" was "hugely inefficient", the President added.

He stressed, however, that the referendum was a decision for the British people and his comments were "not a threat" or an attempt to "fix any votes".

President Obama was speaking after his article in the Daily Telegraph urging the UK to stay in the European Union triggered a furious row.

President Obama again made no bones about his views as he faced reporters at the Foreign Office. 

He said the matter was of of "deep interest" to the US and that the UK was at its best when helping lead Europe.

"It leverages UK power to be part of the European Union. I don't believe the EU moderates British influence in the world, it magnifies it," said President Obama.

David Cameron told reporters "it surely makes sense to listen to what our friends think, to listen to their opinion".

He also pointed to the UK's leading role in negotiations between the EU and the US on setting up the "biggest trade deal in the whole world".

President Obama's intervention left Leave campaigners, including Mayor of London Boris Johnson, in disarray.

Mr Obama began Friday with lunch at Windsor Castle with the Queen and Prince Philip - but he began his day listening to a very different Prince indeed.

They were driven into the courtyard by the 95-year-old Duke of Edinburgh - a ride President Obama later described as "very smooth".

This evening, the President will attend a dinner hosted by the Duke and Duchess of Cambridge.