Barack Obama has warned that Britain's influence on the global stage would be diminished if it votes to leave the EU.
The US President said while cooperation between British and US intelligence agencies would not be affected by an 'out' vote in the referendum on June 23, it would affect the country's global standing.
Speaking to the BBC, Mr Obama said:
He also hit back at critics who accused him of threatening to put the UK "at the back of the queue" for a trade deal with the US if the country votes to leave the EU.
He said he had simply been addressing the "practical" problems of assuming America would fast-track a deal with the UK.
Mr Obama said it could take up to a decade for a US-UK deal to be negotiated - despite the 'special relationship'.
He added: "If I am a business person, or a worker in Britain, and I'm looking at the fact that I already have access seamlessly with a massive market - one of the wealthiest markets in the world, which accounts for 44 per cent of my exports - the idea that I'm going to be in a better position to export and trade by being outside of that market and not being in the room setting the rules and standards by which trade takes place, I think is erroneous."
During the interview, Mr Obama criticised the idea that by cutting itself off from Europe, the UK could solve all its problems.
"Whether you like it or not, we are in an interconnected world. This relates to Brexit, this relates to Nato, this relates to the migration crisis, it relates to our counter-terrorism effort, it relates to public health issues like Ebola," he said.
"It's a problem in the United States - people talk about immigration from places like Mexico.
"It would be tempting for a lot of people to believe that we can pull up the drawbridge. That we can carve a moat around ourselves and not have to deal with problems around the world."
He said he hoped his view would help persuade some voters to choose to remain within the EU.