Home secretary Jacqui Smith addresses the media outside 10 Downing Street on Friday. (AFP)

London, June 29 (Reuters): Radical Islamists may be behind a London car bomb packed with petrol, nails and gas canisters that police defused today, but it is too early to point the finger for certain, security sources said.

Some said the nature of the device and the details to emerge so far — including reports that witnesses had seen the car driven “erratically” — were at odds with al Qaida’s usual mode of operation, which involves disciplined operatives staging multiple, simultaneous attacks.

“Numerically speaking, al Qaida is a strong possibility,” one security source said, in a reference to the estimated 1,600 Islamist militants in Britain whom the MI5 intelligence agency says it is tracking.

“But it just is too early to attribute (responsibility) specifically,” the source said. “It would be wrong and short-sighted to rule out domestic options as well. Obviously there are various extremist groups out there too.”

Dissident Northern Irish republicans might fall into that category, but security analysts saw these as less likely suspects.

“I have very serious doubts it could be the IRA or even a splinter group of the IRA because the IRA’s been successful in negotiating power-sharing in Northern Ireland. Why revert back to a bombing campaign? It’s counter-productive,” said Bob Ayers of the Chatham House think-tank.

A police source said the bomb did not bear the hallmarks of al Qaida.

Scotland Yard said the alarm was raised by an ambulance crew, attending an incident in a nearby nightclub, who thought they saw smoke inside the car. Experts said this could indicate the device had been abandoned after becoming unstable.

Peter Neumann of King’s College, London, said if Islamists were responsible, they may turn out to be self-taught individuals rather than highly trained al Qaida cadres.

“If this is some sort of al Qaida-related thing, it looks very basic, to be honest,” he said. “It doesn’t look like the sort of sophisticated device they would be using. It looks more like 1970s IRA to me.” He added:“If it turns out to be an Islamist-inspired attack, I would probably guess it has been carried out by so-called ‘self-starters’.”