US President Barack Obama has arrived in Britain, where he is set to wade into the poisonous Brexit debate and have lunch with Queen Elizabeth II.
Air Force One touched down at London Stansted Airport, northeast of the British capital, at around 9:45 pm (local time), beginning what is Obama's fifth and likely final presidential visit to the kingdom.
The president, along with First Lady Michelle Obama, will pay tribute to the queen, who turned 90 yesterday, when they meet at Windsor Castle, west of London.
"The president has very much enjoyed his engagements with the queen over the years," said top Obama foreign policy aide Ben Rhodes.
The queen has reigned since 1952, during which time she has met with a string of US presidents from Harry Truman to Obama, who leaves office in January.
The popular monarch has little influence on politics despite her role as head of state, but over the decades she has been a figurehead and ballast for Britons navigating imperial decline, the Cold War and political tumult.
During birthday celebrations Thursday, Prime Minister David Cameron hailed her as "a rock of strength for our nation".
Meanwhile her eldest son and heir Prince Charles said she was held in "love and affection" across Britain and the wider Commonwealth.
After lunch at Windsor Castle on Friday, Obama will travel to Cameron's Downing Street office for talks that come ahead of a crunch British referendum on European Union membership on June 23.
Obama is sure to be asked to weigh in on the controversial issue during a joint press conference or at a town hall-style meeting with youngsters on Saturday.
Britain's departure from the EU - a so-called Brexit - could have deep ramifications for Washington's "special relationship" with London, and on the stability of the 28-country bloc itself.
Obama has consistently said he favours a strong Britain in a strong EU.
Seen from Washington, Cameron's decision to call a referendum was a bold - if not downright risky - gamble that could leave Britain and the EU badly weakened.
"Obama is not an instinctive pro-European," said Ian Bond of the London-based Centre for European Reform think-tank.
"He opposes Brexit because it risks creating more problems for America in Europe."
Polls put the pro-EU and Brexit camps neck-and-neck among those who express a preference to vote, although there is a large pool of people who remain undecided.
Obama's "focus is on how Brexit would affect Europe's ability to help America tackle international problems," said Bond.
