The heads of MI6, MI5 and GCHQ, the signals eavesdropping centre, suggested that while the most likely targets were American or Israeli, there could be British casualties. Their warning was included in a report sent to Tony Blair and other senior Cabinet Ministers on July 16. But the agency chiefs admitted the “timings, targets and methods of attack” were not known.

The disclosure was made yesterday in the annual report of the parliamentary Intelligence and Security Committee, which questioned the intelligence chiefs after the attacks. The July 16 warning to ministers was included in the weekly precis of intelligence assessments made by the Cabinet Office Joint Intelligence Committee (JIC), on which the heads of the intelligence agencies sit.

The JIC prediction of an al-Qaeda attack was based on intelligence gleaned not just from MI6 and GCHQ but also from US agencies, including the CIA and the National Security Agency, which has staff working jointly with GCHQ.

The CIA sometimes has a representative on the JIC. The contents of the July 16 warning would have been passed to the Americans, Whitehall sources confirmed.

The news is consistent with what is now known about warnings given by American agencies before September 11. President Bush was given a CIA briefing on August 6 about a possible terrorist hijacking but the final pieces in the intelligence jigsaw — when, where and how — were missing. Subsequently it has emerged that an FBI agent’s warning of Arab suspects taking flying lessons was ignored.

The Intelligence and Security Committee, headed by Ann Taylor, the former Leader of the Commons, said that the JIC assessment was “not a stark warning of immediate danger to the UK”.

However, the July 2001 JIC assessment, warning that “organised attacks were in their final stages of preparation”, predicted that “UK interests were at risk, including from collateral damage in attacks on US targets”. Seventy-eight Britons died in the attack on the World Trade Centre.

Yesterday Richard Perle, the former US Assistant Defence Secretary, said in London that Britain was exposed to terrorist attack “more than anywhere else other than the US. Britain is a very open place with a large population from which terrorists can be recruited”.

In examining Britain’s preparedness for an al-Qaeda attack last year, the committee report said “the shortage of specific intelligence and Osama bin Laden’s record could have warned all concerned that more urgent action was needed to counter this threat”. The eight MPs and one peer said it was a “matter of conjecture” whether this would have forestalled bin Laden’s actions. But they noted that all three agencies had suffered cutbacks in funds and staffing in the 1990s and had been “operating under financial pressures prior to the September 11 attacks”.

Mrs Taylor said that there had been “intelligence gaps”, and one problem was that the agencies had not envisaged the scale of the September 11 attack. The committee report concluded: “With hindsight, the scale of the threat and the vulnerability of Western states to terrorists with this degree of sophistication and a total disregard for their own lives was not understood.”

The committee, which oversees the work of MI6, MI5 and GCHQ and always takes evidence in private, said: “The questions are whether the threat posed by UBL (Osama bin Laden) was understood and whether it was effectively brought to ministers’ attention.” Before September 11 the security and intelligence services had “identified the pressing need” to gather intelligence about bin Laden and al-Qaeda — a “notably hard target” to penetrate — and had informed ministers “that action was in hand”. The report, the first parliamentary assessment of intelligence leads before September 11, said: “The agencies have told us they had no intelligence forewarning them specifically about the September 11 attacks on the US.”

Sir Stephen Lander, Director-General of MI5, had told the committee seven weeks after the attacks that a subsequent re-examination of material did not find any that, with the wisdom of hindsight, could have given warning of the attacks. John Scarlett, who took over as chairman of the JIC a few days before the attacks in America, told the committee, according to the report, that there was “an acute awareness in the period before September 11” that bin Laden and his associates “represented a very serious threat” and that there was “planning activity”.