British intelligence played a central role in the undercover operation to foil an underwear bomb plot involving al Qaeda's Yemeni offshoot, counter terrorism sources told Reuters.

The undercover informant in the plot linked to Al Qaeda in the Arabian Peninsula, or AQAP, was a British citizen, possibly of Saudi origin, the sources said on condition of anonymity. The informant was working in cooperation with Britain's two principal spy agencies.

U.S. officials revealed publicly on Monday that AQAP tried to arm a suicide bomber with a non-metallic device that was an upgraded version of an 'underwear bomb' that was carried on to a Detroit-bound plane on Christmas Day 2009, but failed to detonate.

The device that authorities seized in the undercover operation contained what was intended to be a more reliable detonating mechanism, counter terrorism officials said.

U.S. officials said earlier this week that the latest plot was foiled by the CIA and allied foreign intelligence services, without identifying the allies.

British authorities put heavy pressure on the Obama administration not to disclose Britain's role in the investigation.

Several U.S. media outlets reported that Saudi Arabia was the key partner in the operation.

But it turns out that British intelligence played what appears to be a more central role in foiling the plot to send a suicide bomber on to an airplane.

The operation was a cooperative venture between Britain's domestic and foreign intelligence services known as MI5 and MI6, officials told Reuters on condition of anonymity.

As details of the operation emerge, it appears to be a striking example of how U.S., European and Middle Eastern intelligence services cooperate on complex counter-terrorism missions.

A spokesman for Britain's Foreign Office declined to comment, saying that in such cases it never confirmed or denied the involvement of British intelligence. A spokesman for British intelligence also declined to comment.

LEAK INVESTIGATION

Several U.S. officials said the operation was severely disrupted when leaks of some of the details began to surface.

Some officials said they believed the operation could have continued for at least another week or two if the leaks to the media had not occurred. They said the leaks could have compromised operational security.

Once the leaks began to surface, authorities began to wind up the undercover operation and the explosive device was handed over to the FBI, which is examining it at its lab at Quantico, Virginia.

Some officials expressed concern that there may be other prototypes of the improved underwear bomb in circulation.

But U.S. officials said they were unaware of any current specific active plots or threats directed against the United States.

U.S. officials said they believed that there was no connection between the discovery of AQAP's latest efforts to launch underwear bombers and the one-year anniversary earlier this month of the U.S. commando raid in which al Qaeda founder Osama bin Laden was killed at a hideout in Abbottabad, Pakistan.

U.S. officials say the seized underwear bomb bears the hallmarks of fugitive Saudi militant Ibrahim Hassan al-Asiri, suspected of being a master bomb-maker working with AQAP.

"He is very dangerous, he is smart, he is vicious. He's the type of person that would put his own brother's life at risk for the cause," Representative C.A. 'Dutch' Ruppersberger, the senior Democrat on the House intelligence committee, said.

"These are the type of individuals that get up in the morning and they are very smart, they are technical, and their whole focus is to attack the United States or our allies," he said.

"So he's an enemy and we have to focus on him and we have to find him."

Ruppersberger would not comment on British involvement in the operation to foil the plot, saying only: "It was a team effort with different countries."

The director of National Intelligence opened an 'internal review' of U.S. intelligence agencies to determine whether there were leaks of classified information related to the underwear bomb operation.

Separately, the FBI is conducting a separate criminal investigation into leaks, a law enforcement official said.

Defense Secretary Leon Panetta, a former CIA director, condemned the leak of classified details about the Yemen operation, saying: "I am fully in favor of a full and thorough investigation of this matter."

Both Panetta and Ruppersberger said leaks of classified information can hamper intelligence gathering.

"When these leaks take place, I can't tell you how much they damage our ability to be able to pursue our intelligence efforts," Panetta told reporters at the Pentagon.

"We are always concerned about leaks, leaks hurt our country, people could get killed, it hurts our credibility where people might not want to give us information or work with us because they are concerned that it will get out," Ruppersberger said.