The special relationship between the United States and the UK remains “so crucial to all of us,” the acting US ambassador has said on the 20-year anniversary of the 9/11 attacks.

Philip T Reeker said the US would be “forever grateful” for the “enduring friendship” and highlighted the importance of unity between the two countries.

A special changing of the guard ceremony during which the US national anthem was played took place at Windsor Castle on Saturday to mark the anniversary of the atrocity.

Hundreds of people gathered inside and outside the walls of the castle to watch as the Band of the Welsh Guards processed through the gates then struck up the Star Spangled Banner.

The anthem was played at Buckingham Palace 20 years ago on September 12 2001, in solidarity with the victims of the atrocity.

Saturday’s rendition was followed by a minute’s silence.

Mr Reeker said the ceremony was a reminder that the US had “no closer ally” than the United Kingdom.

“It’s an incredible privilege to represent my country today on this sombre anniversary but at this ceremony,” he told the PA news agency.

“We’re incredibly grateful to her Majesty the Queen for the playing of our national anthem.

“(It) represents the friendship and solidarity between our two countries, the United States and the United Kingdom, and united is the key word.

“This has shown the special relationship remains so crucial to all of us.

“Speaking for the United States, we have no closer ally and no closer friend in good times and in bad times, and we are very much reminded of that today … through the enduring relationship between our two countries.”

He added: “United is a word I underscore in United States, and United Kingdom.

“We realise we have to face challenges together and we’ve done that for more than three-quarters of a century.”

Some 67 Britons were killed during the attack which claimed almost 3,000 lives.

“My thoughts today are, of course, with all 3,000 of those who we lost in the attacks … including each and every one of the British citizens who were killed,” said Mr Reeker.

“What is important is that our solidarity with allies, particularly the United Kingdom, is on display today.

“We will face challenges as we have before and certainly will do in the future, and we do that together, united in our own friendship, and for that enduring friendship we will be forever grateful.”