By Stefano Ambrogi

LONDON (Reuters) - The British government has warned the aviation industry of a possible al Qaeda attack, security sources and the BBC said on Friday.

The BBC said it had obtained a letter from the Department of Transport addressed to the air transport sector which explained the reason for heightened threat.

"There are indications that al Qaeda may be considering an attack against a UK airport or aviation sector target," it quoted the letter as saying.

"The economic, political and psychological significance of the UK aviation sector coupled with the large crowds present within some of its major assets would enable a successful attack to fulfil al Qaeda's objectives."

Counter-terrorism advice to the transport sector had updated the threat to "severe", meaning an attack was considered highly likely, a British security source told Reuters, although there was no intelligence of any imminent assault.

Britain's national threat level has been at severe for a year and officials played down the significance of the change for airports and railway terminals, indicating it was a precautionary measure.

The BBC said the letter emphasised that no changes to security measures had been made as a result and that the planned method of attack was unclear.

NO U.S. THREAT

An American intelligence official said U.S. authorities were aware Britain had raised its threat alert level for transport hubs but added there was little indication of any increased threat to the United States.

The official, speaking on condition of anonymity, told Reuters that British authorities had taken the action based on recent intelligence reporting which suggested increased threats against transport targets but was vague regarding timing or tactics of any possible attack.

A senior Iraqi official said last month that he believed al Qaeda was planning attacks in the United States, Britain and Europe around Christmas, one year after a failed attempt to bomb a U.S.-bound airliner.

"The government has made no secret of the fact there's an ongoing threat of a Mumbai-style attack by al Qaeda which has been threatened across Europe," a British government source said.

"Terrorists are forever talking about their aspirations to attack targets of significance in the Mumbai-style scenario that involves any crowded place which is what airports and railway terminals are."

British police and security services have investigated dozens of suspected plots and arrested hundreds of suspects since the September 11, 2001 attacks on the United States.

Fifty-two people were killed in London in 2005 when four British Islamists blew themselves up on underground trains and a bus, and the last attempted attack occurred in 2007.

Britain's Department of Transport declined to comment on the latest report, as threat levels for specific national infrastructure sectors are not normally made public.

The Home Office (interior ministry) said that if there was any intelligence of an imminent threat or a plot under way the threat level would be raised to "critical", its highest level.

"We've been at severe since last January and we've been policing at severe. We will police all the key sites and other places in London to that threat level," a police source told Reuters.

"Threat levels can go up and sometimes they can go down. But we are policing at severe and there isn't a threat level higher than severe at the moment." (Additional reporting by Keith Weir, Michael Holden and William Maclean in London and Mark Hosenball in Washington; editing by Andrew Roche)