Senior Cabinet ministers have rejected a report's claims that supporting the invasion of Iraq put the UK more at risk from terrorist attack.
Straw: The time for excuses for terrorism is over
The Chatham House and Economic and Social Research Council paper says the Iraq war has boosted al-Qaeda.
UK involvement in operations against Osama Bin Laden's network has also raised the attack risk, it adds.
But Foreign Secretary Jack Straw and Defence Secretary John Reid said there had also been attacks before the war.
The report, which comes less than two weeks after the London Tube and bus bombings, said the UK's anti-terrorist efforts had been focused on Northern Ireland.
It said the Iraq invasion, in which the UK had been "pillion" passenger, had damaged the counter-terrorism campaign.
Instead it had boosted support, training and fund-raising for al-Qaeda.
Prime Minister Tony Blair has consistently insisted linking the London attacks to British involvement in Iraq or Afghanistan was wrong.
But the report said: "There is no doubt that the situation over Iraq has imposed particular difficulties for the UK, and for the wider coalition against terrorism."
It also stated Britain's "international intelligence, police and judicial co-operation" in operations against al-Qaeda had put it at risk.
Mr Blair's official spokesman said: "Iraq was under the thumb of Saddam Hussein who killed hundreds of thousands of Iraqis. Afghanistan was under the thumb of the Taleban.
"Is the report saying it was a mistake to allow Afghanistan and Iraq to exercise their democratic rights?
"These are the hard questions the report does not address."
And Mr Straw told reporters ahead of a meeting of European Foreign Ministers in Brussels that "the time for excuses for terrorism are over".
"The terrorists have struck across the world, in countries allied with the United States, backing the war in Iraq, and in countries which had nothing whatever to do with the war in Iraq.
"They struck in Kenya, in Tanzania, in Indonesia, in the Yemen, they struck this weekend in Turkey which was not supporting our action in Iraq.
"It is the terrorists who will seek any excuse whatsoever for their action and it is the responsibility of people in the civilised world to stand up to that terrorism and not provide them with any excuse whatsoever."
Mr Reid argued that "the terrorists will kill anyone who stands in the way of their own perverse ideology".
He told BBC Radio 4's Today programme: "I do not accept, when the report says we have made ourselves more of a target because of involvement in Afghanistan and Iraq and our efforts to tackle al-Qaeda, that there is another alternative which is easier and better.
"And the idea that somehow by running away from the school bully, then the bully will not come after you is a thesis that is known to be completely untrue by every kid in the playground and it is also refuted by every piece of historical evidence that we have.
"Terrorism goes way back to the late 1980s and the early 1990s."
The report said UK security services only recognised Islamic terrorists as a major threat in the late 1990s.
Until the July, the report said the UK underestimated terrorists
Before then, groups had been able to operate in London with "relative impunity".
"In an open society, such as the UK, it is notoriously difficult to prevent no-warning co-ordinated suicide attacks, the characteristic modus operandi of al-Qaeda ," the report states.
"The attacks on the transport system in London on 7 July represent precisely the nature of the threat from international terrorism that the UK authorities have been concerned about since 9/11."
It warned terrorists could try to get hold of nuclear, chemical or biological weapons.
"Their track record shows that they would have no compunction about using this type of weapon to cause large numbers of civilian deaths," the report added.