London, April 24 (Reuters): Britain could have to wait a decade for a free trade deal with the United States if it votes to leave the European Union, US President Barack Obama said today in his final salvo of a disputed foray into British domestic politics.

Obama has spent the last three days in London urging Britons to stay in the EU as the British public prepares to vote on whether to remain a member of the bloc on June 23. He played on the two countries' close historic ties to warn that from a US perspective, leaving the EU would be a mistake.

"It could be five years from now, 10 years from now before we're actually able to get something done," Obama told the BBC when asked about the prospect of a post-Brexit trade deal.

That added to his warning on Friday that Britain would find itself at "the back of the queue" for a new trade deal with the US if it departed the EU.

Obama's decision to intervene in the EU debate has angered the eurosceptic "Out" campaign, which has argued that Britain could easily negotiate international deals and get better terms outside the 28-nation EU.

His high-profile warning built on other recent major interventions highlighting the negative economic impact of an exit, with polls showing largely deadlocked public opinion may be starting to shift in favour of the "In" campaign.

Obama's image was quickly added to pro-EU campaign posts online under the slogan "Obama thinks the UK is Stronger In Europe".

London mayor Boris Johnson, de facto head of the "Out" campaign, heavily criticised Obama's comments on trade.

"It is ridiculous to warn that the UK will be at the back of the queue for a free trade deal," he told the Mail on Sunday newspaper. "The UK has never been able to do a free trade deal with the US in the last 43 years - because we are in the EU!"

Answering such criticism, Obama said that his involvement had been justified because of the two countries' longstanding special relationship, and that he hoped he had been able to persuade some British voters.