The continuing debate about the wisdom of Britain's military commitments has intensified after the bombings of three London underground trains and a bus on 7 July which killed 55 people, including the four bombers, and injured some 700 others.
The Royal Institute of International Affairs and the Economic and Social Research Council said the situation in Iraq had given "a boost to the al-Qaida network's propaganda, recruitment and fund-raising" and provided an ideal training ground for al-Qaida-linked groups.
Britain's Foreign Secretary Jack Straw quickly went on the offensive, denying that the UK's support for the US in its war on Iraq had made it more vulnerable to attacks.
"I'm astonished that Chatham House is now saying that we should not have stood shoulder to shoulder with our long-standing allies in the United States," he said, upon arrival at a meeting of EU foreign ministers in Brussels.
"The time for excuses for terrorism is over," Straw said. "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."
Other British ministers, including the Defence Secretary John Reid, also reacted to the report, arguing that terrorism had to be confronted.
"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," Reid said in a BBC radio interview.
The Sunday Times reported that one suspected bomber, 30-year-old Mohammad Sidique Khan, was investigated last year by MI5, Britain's domestic intelligence service, but was not regarded as a threat to national security or subsequently put under surveillance.
The four suspects were all born
MI5 began evaluating Khan, a Briton of Pakistani descent, during an inquiry that focused on an alleged plot to explode a large truck bomb outside a target in London, the newspaper said.
The Metropolitan Police and a spokesman for Prime Minister Tony Blair declined comment.
The bombings have prompted the government to propose new legislation outlawing "indirect incitement" of terrorism -including public praise for those who carry out attacks.
Nevertheless, Charles Falconer, the lord chancellor, denied the government had been negligent in screening political refugees from Muslim countries, making Britain a fertile recruiting ground for Islamic terrorism.
"In terms of asylum, our policy is: If you are in fear of persecution, you are entitled to come here," he said on BBC television. "Obviously, if you then seek to attack the very state that you come to, that gives rise to different questions."
Three of the four men identified by police as the suspected bombers were born in Britain to parents of Pakistani origin.
"If you are in fear of persecution, you are entitled to come here. Obviously, if you then seek to attack the very state that you come to, that gives rise to different questions"
The fourth suspect is Jamaican-born Germaine Lindsay, 19, who came to Britain as an infant and lived in Luton, a city north of London.
Police on Saturday released an image captured by surveillance cameras showing all four bombers with backpacks entering the Luton train station on the morning of the attacks.
Investigators say the four took a train from Luton to London's King's Cross station, where they split up to carry out the bombings.
Officers have also been searching the Leeds home of an Egyptian biochemist after investigators reportedly found traces of explosives in the man's bathtub.
Magdy Mahmoud Mustafa el-Nashar is being interrogated by Egyptian authorities, who say the biochemist denies having any connection to the attacks.
Cairo is not prepared to hand el-Nashar over to Britain, Egyptian security officials said. British investigators are in Cairo to observe the questioning. The two countries have no extradition treaty.