(Adds details, British comment)

By Randall Mikkelsen

WASHINGTON, April 9 (Reuters) - Al Qaeda planner Abu Obaidah al Masri, a main suspect in the 2005 London public transit bombings and a foiled 2006 plot to blow up passenger planes, is believed dead from natural causes, U.S. and British officials said on Wednesday.

"There is compelling reason to believe that Abu Obaidah is dead," a U.S. counterterrorism official said on condition of anonymity. McClatchy newspapers reported that Masri died of hepatitis in Pakistan.

The U.S. official said Masri appeared to have died of natural causes, and a British official said his death had been known to security sources for some time.

"He was a major operational figure," another U.S. official said of Masri, the nom de guerre of one of the least known major al Qaeda figures.

Masri was an Egyptian, known as an explosives expert and a key figure in spreading Islamic militancy to Europe by bringing young Muslims with Western backgrounds to Pakistan for training, said M.J. Gohel of the Asia-Pacific Foundation in London. "The terror trail keeps leading back to Pakistan, and Masri was an essential part of al Qaeda's headquarters in that country."

The U.S. official confirmed that Masri was suspected in the plot to blow up airliners over the Atlantic Ocean. The Washington Post said in 2006 he was believed to be al Qaeda's conduit to British-Pakistani cells that carried out the July 7, 2005 subway and bus bombings in London that killed 56 people.

"He was someone ... who had ties to operations outside of the South Asia region. Al Qaeda lost something when this man died," the U.S. official said, noting, however, that the organization does have a "regenerative capability."

Brookings Institution terrorism analyst Daniel Benjamin said, "If there is any one lesson from the last few years, al Qaeda doesn't have a shortage of operational planners."

British officials declined to comment on links with the London bombings or the 2006 airline bomb plot, for which eight men went on trial last week.

Prosecutors have told the trial that at least four of the defendants visited Pakistan in 2005 and 2006, including a suspected ringleader who went there just a few weeks before the group was arrested. However, they have not provided details of any of the men's alleged contacts there.

U.S. officials declined to discuss Masri's whereabouts when he died. Much of al Qaeda's leadership is believed to be holed up in remote areas of Pakistan near the Afghanistan border.

Masri had been reported killed in a 2006 missile strike in Pakistan, but that was disproved. Later that year he was reported to have escaped a separate missile strike on an Islamic school in Pakistan. (Additional reporting by Mark Trevelyan in London; editing by Chris Wilson)

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