The warnings came at a time when British intelligence concluded there were no imminent attacks planned and lowered the threat alert.
A classified report presented by French intelligence services to the French Interior Ministry in late June, extracts of which were published Tuesday in Le Figaro, said Britain "remains threatened by plans decided at the highest level of al Qaida ... They will be put into action by operatives drawing on pro-jihad sympathies within the large Pakistani community in the U.K."
Three of the four July 7 London bombers were Britons of Pakistani origin.
French authorities have refused to disclose whether Britain was informed of the report.
Saudi Arabian authorities said they informed Britain of a potential attack. The Saudi Ambassador in London Prince Turki al-Faisal said in a statement: "There was certainly close liaison between the Saudi Arabian intelligence authorities and British intelligence some months ago, when information was passed to Britain about a heightened terrorist threat to London."
Security sources told the Guardian newspaper there was no detailed intelligence that could have disrupted the July 7 bombers, who set off devices in three Underground trains and a bus, killing more than 50 people.