BARACK Obama has delivered the starkest of warnings to British voters about what Brexit would mean by making clear that the UK would be put “in the back of the queue” if it wanted to agree a new trade deal with America after leaving the European Union.

The US president acknowledged the June 23 referendum was a "decision for the people of the United Kingdom" and that he was "not coming here to fix any votes".

But, in a joint Downing Street press conference with David Cameron, he stressed how being a good friend meant being honest and that the future of a key ally’s relationship with a major trading bloc was of interest and importance to the US.

"Particularly because my understanding is that some of the folks on the other side have been ascribing to the United States certain actions we will take if the UK does leave the EU; they say, for example, that 'we will just cut our own trade deals with the United States'.

"So they are voicing an opinion about what the United States is going to do; I figured you might want to hear from the president of the United States what I think the United States is going to do.

"And on that matter…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.”

Mr Obama then declared: "The UK is going to be in the back of the queue," noting how trying to do piecemeal trade agreements was "hugely inefficient".

The point is a major stab at the heart of the Leave campaign. Only this week, leading Out campaigner Michael Gove, the Justice Secretary, insisted by exiting the EU Britain would be able to take control of trade negotiations and seal deals with the likes of America “more quickly”.

During the hourlong press conference, the president did not hold back in his intervention in the EU debate, stressing how membership of the Brussels bloc made the UK a "bigger player" on the world stage and enhanced its "special relationship" with the US because it meant Washington had a trusted partner in Europe on issues including the fight against terrorism.

He explained: "Precisely because I have a confidence in the UK and I know that if we are not working effectively with Paris or Brussels, then those attacks are going to migrate to the United States and to London, I want one of my strongest partners in that conversation.

"So it enhances the special relationship, it does not diminish it."

A clearly delighted Prime Minister made clear the referendum was the "sovereign choice of the British people" but added: "It surely makes sense to listen to what our friends think, to listen to their opinion, to listen to their views and that's what Barack has been talking about today."

In response, Richard Tice, co-founder of the Leave.EU campaign, said Britain did not have a trade deal with the US because it was in the EU and added: "Obama doesn't have the authority to deny us a deal as he will be long gone before any such proposals are on the table."