The former head of the UK Armed Forces has said the return of the Taliban in Afghanistan raised the prospect of "another 9/11".
General Lord Richards said this was due to ungoverned spaces opening up – which the terrorists were able to exploit.
"I think we are (closer to another 9/11)," he told LBC.
"We've now been pitched back into a dark period which we somehow have to manage."
His comments came as the Queen, Boris Johnson and Sir Keir Starmer marked the 20th anniversary of the September 11 attacks by praising the international resolve that has grown from the al-Qaeda-led tragedy.
In a defiant message to be played at a memorial event at the Olympic Park in east London on Saturday, the Prime Minister said the threat of terrorism remained but people refused to live their lives in "permanent fear".
"The fact that we are coming together today – in sorrow but also in faith and resolve – demonstrates the failure of terrorism and the strength of the bonds between us," Mr Johnson said.
The Queen remembered her visit to the site of the attack.
In a message to the US President Joe Biden, she said: "As we mark the 20th anniversary of the terrible attacks on September 11 2001, my thoughts and prayers, and those of my family and the entire nation, remain with the victims, survivors and families affected, as well as the first responders and rescue workers called to duty.
Watch: 'Which Movie Is This?' – how British veteran remembers 9/11 reaction.
"My visit to the site of the World Trade Centre in 2010 is held fast in my memory.
"It reminds me that as we honour those from many nations, faiths and backgrounds who lost their lives, we also pay tribute to the resilience and determination of the communities who joined together to rebuild."
Labour leader Sir Keir Starmer said the consequences of the attacks were "still being felt to this day", adding that the tragedy was "still so raw".
He said: "But as we mark this anniversary I'm convinced our resolve has never been stronger.
"We will continue to fight terror and violence, by promoting our values of justice and peace."
Mr Johnson said recent events in Afghanistan had only strengthened people's belief in freedom and democracy.
The political leaders' comments came as the prime minister at the time of the attacks – Tony Blair – said the international community must be prepared to take action against the Taliban if they again allow Afghanistan to become a base of terrorism.
Mr Blair said the US and its allies had no choice but to invade after the Taliban refused to give up the al-Qaeda leadership responsible for the attacks.
Cover image: The World Trade Center complex continues to burn, while the Coast Guard Cutter Tahoma controls vessel traffic in New York harbour on 15 September (Picture: US Department of Defense).