LONDON, April 25 - Crime and disorder similar to Britain's 2011 summer riots are the most likely serious threats to the London Olympics although Islamist militants and al Qaeda offshoot groups pose a growing challenge, a British official said on Wednesday.

Charles Farr, Director-General of the Office for Security and Counter-Terrorism at the Home Office (interior ministry), told a security conference that while al Qaeda's south Asia-based core leadership had weakened following the death of Osama bin Laden in 2011, the clout of its offshoot groups was rising.

In a speech surveying trends in global counter-terrorism, Farr added that al Qaeda remained active in Britain, and the operating model of the group's leadership was increasingly to encourage "self starter" terrorists in recognition of its own inability to exercise operational control.

But in a passage on security for the Olympics, Farr added: "The most likely serious threat is crime and public disorder."

He said the variety of disorder he envisaged as a potential threat was the rioting that swept England's cities last year. He did not elaborate.

Up to 15,000 people took part in England's worst disturbances for decades, which began in inner London when a protest against the fatal police shooting of a suspect turned violent. Unrest spread to other major cities such as Manchester, Birmingham and Bristol, leaving a trail of torched buildings and looted shops in their wake. Five people died in the unrest.

An independent investigative panel identified a series of problems facing inner cities, ranging from poor parenting and education to high joblessness that left many people with no stake in society and nothing to lose if they joined the riots.  

The security situation in London will be under international scrutiny in July and August this year when tourists and sports fans flood to the British capital for the Olympics.  

Home Secretary Theresa May said on Jan. 25 that along with terrorism and organised crime, disruption from protests was one of the biggest threats to the Olympics which begin on July 27.

Britain's National Olympic Security Coordinator Chris Allison told the Guardian newspaper in February that it would be "very challenging" if, during the Olympics, there were a repeat of the disorder that gripped London last summer.

Allison was quoted as saying that while there did not appear to be anyone who wanted to protest against the Games, there might be those who want to use the Games as a way of getting their cause into the public domain.

Police have insisted they had no intention of preventing any legal demonstrations outside Olympic venues.  

Farr said the Olympics would stretch the integration of the web of institutions and services involved in ensuring security and one "pinch point" was transport, particularly London's, which he said was overloaded at the best of times.

British officials say they have not as yet uncovered any information suggesting any groups are planning to target the Games. But Britain has been a target for Islamist militants for the last decade as an ally for U.S.-led military action in Afghanistan and Iraq and London suffered its worst peacetime attack in 2005 when four suicide bombers killed 52 commuters.