The report's conclusions paint a disturbing picture of the ease with which individuals can plan and execute relatively simple yet deadly attacks, and highlights the difficulties faced by the security services in preventing such acts.

The leak of the government's official account of the bombings led to renewed calls for an independent inquiry Sunday night.

The government has previously resisted such calls, ordering instead a "narrative" of events leading up to the attacks. Compiled by a senior civil servant in the Home Office, the narrative is due to be published within two months, but a leaked draft was obtained by the Observer newspaper.

The report's provisional conclusions are that the bombings were a "simple and inexpensive" plot by four homegrown radicals operating independently from any international terrorist network.

"The London attacks were a modest, simple affair by four seemingly normal men using the internet," a Whitehall source said.

Mohammed Siddique Khan, Hasib Hussain, Shehzad Tanweer and Germaine Lindsay killed 52 people when they carried out four coordinated suicide bombings on the city's transport network. Khan, Hussain and Tanweer were British citizens of Pakistani descent living in or near the city of Leeds, West Yorkshire, while Lindsay was a Jamaican-born British resident who spent his teenage years in West Yorkshire before moving to Aylesbury in Buckinghamshire.

Acting on their own initiative, the group scoured the internet for information on making bombs, which cost just a few hundred dollars to assemble, the report says. It found no evidence to indicate the attacks were masterminded by an al-Qaida "fixer," a theory explored by the security services in the months following the bombings. Although the four suicide bombers may have been partly inspired by a trip by Khan and Tanweer to Pakistan, any meeting between the men and known militants were ideological rather than fact-finding, it concludes.

A videotape of Khan released in the aftermath of the attacks features footage of Ayman al-Zawahiri, said to be Osama bin Laden's second-in-command. But the Home Office report dismisses it as evidence of al-Qaida involvement, suggesting that it was edited at a later date to give a false impression.

Neither was there anything to suggest that a fifth bomber was involved in the plot. After the bombings, police found unused explosives in the bombers' abandoned car at Luton station near London, which prompted a manhunt for a missing suspect. However it now appears the explosives were simply superfluous.

The narrative confirms Khan as the ringleader of the plot, and explores the behavior of the four men in the preceding months. Using intelligence garnered in the nine months since the attacks, it details how the bombers led double lives, adopting an extreme interpretation of Islam yet enjoying a "western" lifestyle.

It depicts the bombers as young men motivated by anger at Western foreign policy, which they perceived to be deliberately anti-Muslim, and lured by the promise of immortality.

The Home Office narrative will increase anxieties over the vulnerability of Britain to inexpensive and unsophisticated attacks conducted by homegrown radicals with no experience or support. Two weeks after the fatal bombings, an apparently unconnected group attempted to duplicate the attacks, but their devices failed to detonate properly and caused no harm.

The narrative will be criticized for failing to explore why the security services terminated its surveillance of Khan after identifying him in connection with a previous plot to attack Britain the year before the July bombings. That question is being addressed by Parliament's Security and Intelligence Committee, also investigating the attacks, which may release its findings alongside the Home Office report.

The BBC reported in March that the committee would clear the security services of negligence in the run-up to the bombings, but would question why Khan was not fully investigated.

Critics said the Home Office narrative had left many questions unanswered, and demanded an independent and public probe.

Conservative Homeland Security Spokesman Patrick Mercer said: "A series of reports such as this narrative simply does not answer questions such as the reduced terror alert before the attack, the apparent involvement of al-Qaida and links to earlier or later terrorist plots."

He said he found the suggestion that al-Qaida had not been involved on any level hard to believe, and said an independent inquiry was necessary to avoid another government "whitewash" such as that over "dodgy" intelligence on Iraqi weapons of mass destruction.

The Home Office said they would not comment on a leaked document nor pre-empt the publication of the official account of July 7.

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