Former premier Tony Blair, who played a key role in responding to the 9/11 attacks, said on Saturday Islamist terrorism remained a deep-rooted problem on the eve of the atrocity's 10th anniversary.
Blair praised Western powers for reducing the terrorist threat but warned leaders should not let their guard down because of the death of Al-Qaeda chief Osama bin Laden, who masterminded the 2001 attacks on the United States.
"I think we have certainly achieved significant results in fighting (Islamist terrorism). I think we have improved our security defences against the terrorists," he told BBC radio.
"I think we have knocked out a lot of the Al-Qaeda network."
But he added: "I don't think this is over. I think the radical Islamism which gave rise to this terrorist group is still with us, is still powerful and still has its roots and its effects in many different parts of the world."
"I think it will take a generation" to eradicate the ideology that was driving people towards terrorism, he added.
Blair, premier from 1997 to 2007, famously said that Britain stood "shoulder to shoulder" with Americans in the immediate aftermath of the attacks and strongly supported then US president George W. Bush's response to the atrocity.
Ceremonies will take place in the United States on Sunday to mark a decade since Al-Qaeda hijackers slammed passenger planes into the World Trade Center, destroying its iconic Twin Towers, and the Pentagon, in the nation's capital.
A fourth plane crashed in a Pennsylvania field when the passengers overpowered the hijackers. Almost 3,000 people were killed that day in the worst attacks on American soil.
The attacks sparked a 10-year manhunt for bin Laden, who was finally tracked down and killed in a covert raid by US Navy SEALs in Abbottabad, Pakistan, in May.
But Blair warned his death did not mean the fight against terrorism was over, saying that "this is not just about one man, this is actually about a terrorist movement".