In an information briefing released on MI5's website after the 7 July attacks the agency said Iraq was a key issue motivating the bombers.
"Though they have a range of aspirations and "causes", Iraq is a dominant issue for a range of extremist groups and individuals in the UK and Europe," said the briefing, which can be found at www.mi5.gov.uk/output/Page269.html.
"Some individuals who support the insurgency are known to have travelled to Iraq in order to fight against coalition forces. In the longer term, it is possible that they may later return to the UK and consider mounting attacks here," the briefing continued.
According to the briefing, MI5 believes that British and foreign nationals who are linked to the al-Qaida network are present in the UK.
The agency's position contradicts that of the British government's, which has repeatedly rejected any link between the attacks and the war on Iraq in which Britain plays a major role.
"We are not having any of this nonsense about it. It's nothing to do with what the British are doing in Iraq or Afghanistan, or support for Israel, or support for America, or any of the rest of it. It is nonsense, and we have got to confront it as that," Blair said on 26 July.
In a recent poll, 85% of Britons said they believed the attacks were linked to the war on Iraq.