A man with lower-than-average intelligence is believed to have prompted the anti-terror raid in London's Forest Gate, during which one man was shot, it was claimed last night.

In what could prove to be highly embarrassing for the police and security services, the 22-year-old man, who has an IQ of just 69, was the trigger behind the dawn raid this month, according to the Sunday Mirror.

The former waiter, a childhood friend of the two brothers who were arrested during the raid two weeks ago, was jailed for six years in January for terror offences.

The paper has been told that it was after friends of Mohammed Abdul Kahar, 23, and Abul Koyair, 20, visited the man in top-security Belmarsh prison that the brothers were put under surveillance.

The brothers reportedly laughed off the fact that they were being followed by the security services. New information passed to the services, said to include detailed drawings of a suicide vest, led to Operation Volga and the early morning moves to arrest the pair, the paper claims.

Friends of the brothers told the paper they believe the man was the trigger for the operation that culminated in 250 police officers descending on their home.

Mr Kahar was shot in the shoulder during the raid in the early hours of 2 June, before being taken for questioning. The pair were released a week later without charge.

The claim is set to pour fuel on the pyre already being assembled around the career of the Metropolitan Police Commissioner, Sir Ian Blair. Calls for his resignation have intensified in the wake of the force's high-profile raid.

Critics, including some of his own officers, politicians and members of the Muslim community, have said that he must step down.Today, Muslims plan to hold a rally in East London in protest at the raids.

A senior source at the Met said yesterday that the commissioner had been making a concerted effort to win over his detractors, and that officers had been told they must be "on message".

"It's rare that you get a really juicy bit of intelligence [as with Forest Gate] and you go on and net a terrorist. Often you must rely on scraps here and there, which you have to glue together," the source added. "We will stifle that flow of low-level intelligence if the focus is all on Ian Blair, then we will lose the lead."

In an interview yesterday, Lord Stevens, the former Commissioner of the Metropolitan Police, called on police to focus on the core issues of fighting crime and not become "political". "I have nothing to say about Ian Blair other than that he was a superb Deputy Commissioner. I wish him all the best. He has got a very difficult job to do," Lord Stevens said.

"I just think we have got to be very, very careful that we concentrate on the core issues of preventing and detecting crime. That's why the Met was set up."

Chief Superintendent Ali Dizaei, from the National Black Police Association, called for the Independent Police Complaints Commission (IPCC), which is investigating the shooting of Mr Kahar, to widen its probe.

"When you speak to the Muslim community, they are extremely concerned about what they have seen on television but, equally, they want an explanation of the officers' actions and of the intelligence," said Mr Dizaei, a borough commander. Lord (Brian) MacKenzie, who advises the Government on police issues, expressed concern about how the police and the security services checked out intelligence they received. "I find it difficult to understand how such a major mistake can be made," said the former chief superintendent.

"The police do seem to have gone over the top. This is something that could have equally happened on John Stevens's watch. But the force would have rallied round him ­ Ian Blair does not seem to have secured the same credibility."

John Denham MP, former police minister and chair of the Home Affairs Select Committee, said that Sir Ian's position was "strong" but was dependent on the outcome of an IPCC investigation into the shooting of Jean Charles de Menezes at Stockwell underground station last year.

FOR AND AGAINST

Hello, hello, hello

The supporters who believe Ian Blair should stay

Ken Livingstone

The Mayor of London has accused the right-wing press of "running a campaign to remove Sir Ian Blair"

Paul Stephenson

The Met's deputy commissioner has said: "He has a very big job to do, which is making London a safer place"

Alan Rusbridger

Guardian editor wrote article entitled "Why Blair must not quit" 12 months after the commissioner took up his post

Goodbye, goodbye, goodbye

The critics who think it is time for him to go

Mike Penning MP

One of three Conservative backbenchers to table a Commons motion calling for Sir Ian's sacking following his remarks about Soham

Peter Smyth

Chairman of the Met's constables branch told the Police Federation conference in May: "We have no confidence in this commissioner"

Shami Chakrabarti

Director of human rights group Liberty has called for Sir Ian's resignation, branding his actions "bizarre, unconstitutional and possibly unlawful"