The intelligence agencies are monitoring and will continue to monitor every Muslim who travels from Britain to Mecca on Umrah or Hajj pilgrimage in a wider effort to piece together intelligence on suspected Al-Qaeda activity.

A senior Whitehall official disclosed that the operation targeting trips to the holy city in Saudi Arabia by more than 100,000 British Muslims is part of a trawl by MI5 and MI6 for information about movements of suspects. It follows claims that British Muslims had visited the city before carrying out alleged terror related activity in Britain and abroad.

The importance of the intelligence operation was one of the reasons given by spy chiefs for maintaining ties with Saudi Arabia when the Saudi government was threatening to break off intelligence ties over a bribery investigation by the Serious Fraud Office (SFO) into BAE, Britain’s prime defence contractor.

Sir John Scarlett, the head of MI6, and Dame Eliza Manningham-Buller, the directorgeneral of MI5, told Lord Goldsmith, the attorney-general, that Saudi co-operation in the fight against Al-Qaeda was vital.

A well-placed security official said Scarlett and Manningham- Buller used the Mecca surveillance operation as evidence of the need for continuing intelligence ties with the kingdom: “They made it clear to Goldsmith that they were concerned about the implications for national security of losing Saudi co-operation. They said that every British Muslim who makes the pilgrimage to Mecca was monitored.”

Muslim leaders voiced their concerns about the operation. Dr Ghayasuddin Siddiqui, leader of the Muslim parliament, said: “It is absolutely wrong that people who are going to Mecca for entirely religious purposes should be monitored by the security services. It is a sad commentary on Britain’s relations with Saudi Arabia.”