The plan, which the French DGSE foreign intelligence service became aware of earlier this year, was revealed in a secret report submitted to the French government.
The report, dated December 19, indicates that the tip-off came from the American CIA. British and French intelligence agencies have run a series of checks of the security system protecting the 31-mile tunnel but the threat level, the DGSE warns, remains high. British security services remain on high alert throughout the holiday period.
Quoting French sources, the newspaper reported that the plan was put together in Pakistan and is being directed from there. The plotters are believed to be Western Europeans, possibly Britons of Pakistani descent.
The DGSE say that levels of "chatter," the constant communication that takes place between militants, has not been so high since 2001.
'Threat is sky high'
Last week, Sir Ian Blair, head of the Metropolitan Police, described 'the threat of another terrorist attempt' as "ever present," adding that "Christmas is a period when that might happen."
"It is a far graver threat in terms of civilians than either the Cold War or the Second World War," he said. "It's a much graver threat than that posed by Irish Republican terrorism."
American security sources also told The Observer that the threat was "sky high."
The news of the threat to the Channel Tunnel comes as Eurostar trains transport record numbers of passengers heading home for Christmas and as fog continues to affect flights to and from the continent.
More than 8 million passengers traveled on Eurostar trains last year.
"A successful attack on such an installation would be almost as spectacular as September 11," a terrorist expert told The Observer. "Al-Qaeda and those they inspire are trying everything from low-level strikes to major attacks on critical infrastructure."
The DGSE report also mentions an al-Qaeda project for a "wave of attacks in an unidentified European country planned and run from Syria and Iraq." The period of highest risk is said to be from September 2006 to April 2007.
Eliza Manningham-Buller, director-general of the MI5 British security service, recently disclosed that UK intelligence services are monitoring more than 200 networks and 1,600 individuals in Britain.