Barack Obama has urged the UK to stick with the EU, as he began his final trip to Britain as US president by having lunch with the Queen.
Mr Obama, who is on a three-day visit with his wife Michelle, is meeting PM David Cameron for talks.
Mr Obama has told the Daily Telegraph being in the EU magnified Britain’s influence across the world.
But London Mayor Boris Johnson called his view “hypocritical”, describing the EU system as “alien” to US traditions.
Prime Minister David Cameron is hosting Mr Obama at Downing Street.
The leaders are holding talks on the fight against so-called Islamic State (IS) and will later be joined by the UK’s home, foreign and defence secretaries, and the chancellor.
Mr Cameron and Mr Obama are expected to discuss the progress being made in combating IS in Iraq and what more can be done to tackle the group in Syria.
The situation in Libya, where the West is considering how to support the newly-established Government of National Accord (GNA) in the battle against IS, will also be discussed.
The pair will later hold a joint press conference.
Earlier the president and First Lady Michelle Obama attended a private lunch with the Queen and Duke of Edinburgh at Windsor Castle – the day after the Queen’s 90th birthday celebrations.
The Obamas brought the Queen a photo album of her many visits with US presidents and first ladies.
The album begins with her visit to the United States in 1951.
Then known as Princess Elizabeth, Duchess of Edinburgh, she toured George Washington’s Mount Vernon Estate and met with President Harry Truman at the White House.
The Duke and Duchess of Cambridge and Prince Harry will welcome the Obamas for dinner at Kensington Palace on Friday evening.