A senior detective who worked on the September 11 investigation has joined calls for a public inquiry into the July 2005 suicide bombings in London.

Detective Superintendent Des Thomas told The Times that an independent examination of the apparent intelligence failures that allowed the 7/7 bombers to strike was essential to prevent a repetition of those mistakes.

Mr Thomas, whose 35-year police career also included IRA and Animal Liberation Front cases, is prepared to appear as a witness on behalf of a 7/7 survivors’ group that is taking the Government to court in an attempt to force an inquiry.

Jacqui Smith, the Home Secretary, rejected the group’s calls for an inquiry last week and Government lawyers told the group that its court challenge was “premature and misconceived”. Ministers, police and MI5 officials oppose an inquiry, claiming that it would divert resources.

But Mr Thomas, now retired from the police, said that the authorities were creating a “smokescreen” and an inquiry could be conducted quickly and efficiently.

The core issue for any inquiry into the 7/7 attacks, in which 52 people died and more than 700 were injured, is why surveillance on the bombers’ leader Mohammad Sidique Khan was apparently dropped in 2004.

Khan, 30, from Dewsbury, West Yorkshire, came to police and MI5 attention 16 months before the bombings, when he was photographed and bugged in the company of a group of men planning to carry out a bomb attack.

When the plotters were arrested in March 2004, Khan was classified as a “peripheral target” and inquiries into his activities were, apparently, discontinued. It is not known what discussions took place between MI5, Scot-land Yard and West Yorkshire police about further investigation of his extremist activity. Mr Thomas, who is regarded as an expert on the conduct of criminal investigations, said: “If Khan did drop off the radar then there was a huge flaw in the way these matters were being investigated. If the proper procedures were being followed, all the decisions relating to Khan should have been properly documented. There should be minutes of meetings between MI5 officials and police of chief officer rank. There should be ‘policy books’ recording every decision and the justification for that decision.

“If these matters were documented then the initial work of an inquiry would not take very long at all. It would take me, or someone of my training, just two days to read all the relevant documents and identify the problems.

“If the documents are not available it would take longer, but in such a scenario the argument for public accountability would be even more powerful than it is now. If the documents do not exist, then either those in charge of the investigation did not know what they were doing or they don’t want people to know what they were doing.”

Mr Thomas, former deputy head of Hampshire CID, said he believed that 7/7 could have been prevented if action had been taken when Khan first came to attention.

He said: “The question which has to be answered is, ‘Was this avoidable?’. Had better management of resources and techniques been in place could it have been stopped? My suspicion is that it was avoidable.” Claims that holding an inquiry could hamper the work of national security were “obfuscation”, Mr Thomas said.

He added: “It is wrong to say you cannot examine terrorist issues because of secrecy - it’s all to do with glamorising the work and people being self-important. Really, terrorism is nothing more than organised crime. Investigations must be managed properly and that all comes down to mind-grinding attention to detail.”

In 2001 Mr Thomas was a senior investigating officer on the 9/11 investigation dealing with the repatriation of the remains of British victims of the attack on the World Trade Centre in New York.

He has lectured at the Police Staff College and at the Institute of Criminal Justice Studies at the University of Portsmouth and Cardiff Law School.

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