US President Barack Obama will formally begin his three-day state visit to Britain today.
Mr Obama and his wife Michelle landed in Stansted in London last night after their trip to Ireland was cut short because of the latest volcanic eruption in Iceland.
President Obama and the First Lady will stay as guests of the Queen at Buckingham Palace tonight and tomorrow night.
Mr Obama declared the UK-US partnership “an essential relationship for us and for the world”.
The state visit – only the third by a US president to the UK in 100 years - comes at a time of close co-operation between Britain and America on issues ranging from Libya to Afghanistan, counter-terrorism and the Middle East peace process.
This strong partnership was reflected in a joint article written by Mr Obama and Prime Minister David Cameron for The Times, in which they declare: “Ours is not just a special relationship, it is an essential relationship – for us and for the world.”
After his early arrival, Mr Obama was spending the night in the US ambassador’s residence in London, Winfield House in Regent’s Park.
Despite the hasty change to their planned schedule, Mr Obama and wife Michelle were still given the formal welcome expected of a state visit.
They were greeted by the Lord in Waiting Viscount Brookeborough, who met them on behalf of the Queen.
Instead of a traditional red carpet they left the plane on special red-carpeted stairs because of windy conditions at the airport.
Today they will be greeted by the Prince of Wales and the Duchess of Cornwall. There will then be a ceremonial welcome for the visitors in the Palace garden from the Queen and Duke of Edinburgh, and the Obamas will view the royal picture gallery before a wreath-laying visit to Westminster Abbey and a state banquet in the evening.
The couple will stay as guests of the Queen at Buckingham Palace tonight and tomorrow.
Tomorrow will be devoted to politics, with talks between Mr Obama and Mr Cameron at 10 Downing Street, followed by an address to both Houses of Parliament, in which the president is expected to say that the US has no closer ally in the world than Britain.
In their joint article, Mr Obama and Mr Cameron put the transatlantic partnership at the heart of the drive for global stability and prosperity.
“When the United States and Britain stand together, our people and people around the world can become more secure and more prosperous,” they wrote.
“And that is the key to our relationship. Yes, it is founded on a deep emotional connection, by sentiment and ties of people and culture. But the reason it thrives, the reason why this is such a natural partnership, is because it advances our common interests and shared values.
“It is a perfect alignment of what we both need and what we both believe. And the reason it remains strong is because it delivers time and again.
“Ours is not just a special relationship, it is an essential relationship - for us and for the world.”
They also promised not to abandon the protesters fighting for democracy in the “Arab Spring”.
“We will not stand by as their aspirations get crushed in a hail of bombs, bullets and mortar fire,” said the two leaders.
“We are reluctant to use force, but when our interests and values come together, we know we have a responsibility to act...
“We will stand with those who want to bring light into dark, support those who seek freedom in place of repression, aid those laying the building blocks of democracy.”