Terror “still knocks at the door of democracy”, Andy Burnham has warned as he expressed the nation’s unity with the US on the 25th anniversary of the 9/11 attacks that shocked the world.

With ceremonies being held either side of the Atlantic on Friday to mark the sombre milestone, the Prime Minister said the UK and US “stand together in memory of those we lost, and in defence of the values we cherish”.

He recalled how in the wake of terrorist outrages in Manchester during his time as mayor of the city region, messages of support had been received from across the US, including the children of those killed on September 11, 2001.

Meanwhile, Sir Tony Blair, who was prime minister at the time of the murderous atrocity and had to deal with its aftermath, said the anniversary was also a time to “commemorate the unshakeable and essential bond” between the two countries.

US ambassador to the UK Warren Stephens said Britain had rallied in support “in one of America’s darkest hours” and that “despite our disagreements and debates”, the transatlantic relationship “has kept us safe for centuries”.

His comments follow strained relations with President Donald Trump’s White House, most notably over the Iran war, which have tested long-standing ties with traditional allies.

It was on a bright, cloudless day 25 years ago that 2,977 people – 67 of them British – were killed when passenger jets hijacked by al Qaida terrorists struck the twin towers of the World Trade Centre in New York, causing both to collapse, and the Pentagon outside Washington DC.

A fourth plane seized by extremists – United Airlines Flight 93 – crashed into a field in rural Pennsylvania after the crew and passengers fought back with the famous line “let’s roll”, and so prevented it from hitting another target, thought to be the congressional building, which houses the US legislature.

It remains the worst terrorist outrage of all time, claiming the lives of victims from more than 90 nations.

In response, the US became the first and only ever country to invoke Nato’s “all for one, and one for all” collective security clause, where members rally to the defence of an ally under attack – a commitment now called into doubt by the current president.

It led to the large-scale deployment of UK forces to conflicts in both Afghanistan and Iraq, which left hundreds of British troops dead, thousands more injured and continue to have global repercussions.

Mr Burnham said: “On the 25th anniversary of 9/11, we remember the nearly 3,000 people who were killed, including 67 Britons.

“My thoughts are with all those who lost friends and family members that terrible day. Time may pass, but I know the grief is still felt just as deeply today.

“On this anniversary, we must remember that hatred and fear did not have the last word on 9/11.

“The terrorists sought to weaken and divide America, and everyone who believes in the values our countries share. But our response was clear: those who seek to destroy our way of life will never win.”

He added: “I saw that same defiance in Greater Manchester. When I was mayor, Manchester was the victim of two horrific terrorist attacks. I will never forget the messages of support and friendship from people across America who knew better than anyone the suffering of our city.

“Some of the children of those killed in the 9/11 attacks wrote to the families of the people killed in Manchester, and they said this: we are you and you are us. We are one.

“We will never forget these terrible events, but we refuse to be defined by them. Terror still knocks on the door of democracy, but the United Kingdom and the United States stand together in memory of those we lost, and in defence of the values we cherish.

“We are you and you are us. And we will prevail.”

Reflecting on the anniversary, Sir Tony said: “I will forever remember 11th of September 2001, as a leader but also as a citizen.

“9/11 was the worst terrorist attack ever perpetrated, the greatest loss of life through an attack on American soil in the history of America and the largest death toll of any act of terrorism in respect of the British people.

“That day changed the world and 25 years on, changes the world even now.”

He added: “We remember those who lost their lives, 3,000 of them on that day. We remember their families. We recall the courage of the responders, many of whom also sacrificed themselves in that moment.

“And we remember the incredible fortitude of the people of New York and the other cities affected. And we recognise that, though thankfully such an attack has never been repeated in America, the struggle against terrorism in all its forms continues.

“We should all recommit ourselves to confront and defeat those who engage in such terrorism, and to pursue the cause of peaceful co-existence between people of all faiths, cultures and nations.

“Here in Britain, as we remember this day, we also commemorate the unshakeable and essential bond between our country and the United States of America.”

Speaking at a ceremony at the American embassy in London to dedicate a memorial stone to the victims of 9/11, the US ambassador said following the 2001 attack “a new chapter was added to the world’s most historic partnership”.

Mr Stephens recalled: “As America grieved, the people of the UK rallied to stand beside us in sympathy and solidarity.”

As well as the public displays of unity, the ambassador said: “Away from the public eye, our military and intelligence communities began to work together on a level unseen in international relations: sharing information, conducting joint operations and disrupting threats in the weeks and months after 9/11.

“That co-operation saved countless lives – and it continues to save lives today. We must always remember the painful lessons 9/11 taught us.

“Because despite our disagreements and debates, the US-UK partnership has kept us safe for centuries: helping us overcome threats from Nazi tyranny to communist ideology to terrorist insurgency.”

He added: “In one of America’s darkest hours, the UK – a nation attuned to grief and terror – rose as one to stand alongside their American brothers and sisters.”