U.S. intelligence agencies and the FBI are conducting investigations to find the source of a leak about the use of a double agent in Yemen who had joined al Qaeda but was serving as an informant for both Saudi intelligence and the CIA. The leak is said to have undermined intelligence operations. What the leak also reveals is that British intelligence agents are now implicated in assassinations — operations that are illegal under British law.

The Guardian reports: A British citizen played a central role in foiling the latest “underwear” bomb plot hatched in Yemen to attack a US-bound plane, as well as in the assassination of a top al-Qaida operative at the weekend, according to various sources in Washington on Thursday.

CNN reported that the agent involved was a British citizen of Saudi origin who had been recruited about a year ago by Saudi intelligence.

MSNBC, which also reported that the agent was a British passport holder, said that British intelligence was “heavily involved”. Other US media outlets gave the Saudi intelligence service most of the credit for the successful running of the operation. The Guardian independently confirmed British involvement.

The agent was recruited by al-Qaida in the Arabian Peninsula (AQAP), which operates in Yemen and Saudi Arabia, and asked to carry a bomb aboard a US-bound plane.

The revelation is politically and legally awkward for MI6 and MI5 whose agents, unlike American ones, are banned from missions that lead to assassinations, such as the US drone attack at the weekend that killed the top al-Qaida operative in the Yemen, Fahd al-Quso. The attack is being attributed to information from the agent.

Such is the sensitivity that America’s National Public Radio reported that the British government asked the Obama administration not to reveal the role of British intelligence in the mission.

James Clapper, the US director of National Intelligence, has opened an “internal review” of US intelligence agencies to determine whether there had been leaks of classified information related to the underwear bomb operation.

The FBI is conducting a separate criminal investigation, a law enforcement official said. The US defence secretary, Leon Panetta, a former CIA director, said: “When these leaks take place, they damage our ability to be able to pursue our intelligence efforts.”