Barack Obama has said that the UK could take up to 10 years to negotiate trade deals with the US if it leaves the EU.
It came a day after he said the UK would be at "the back of the queue" for US trade deals in the case of Brexit.
His comments have angered campaigners for leaving the EU.
Mayor of London Boris Johnson called them hypocritical.
Subsequently, about 35,000 protestors marched in Germany against the partnership.