The Government's conclusions raise concerns over the ease with which relatively unsophisticated suicide attacks can be planned and executed, and it underlines the difficulties the security services face in thwarting such operations.
A leak of the Home Office report brought renewed calls last night for an independent inquiry into the London blasts, which killed 52 innocent people.
Charles Clarke, the Home Secretary, ordered a "narrative" of the events leading up to the detonation of three bombs on London Underground trains and a fourth on a bus.
It is due to be published within two months but a leaked draft, obtained by The Observer, shows the provisional conclusion is that the attacks were a "simple and inexpensive" plot by four suicide bombers acting on their own initiative.
The report says they trawled the internet for information on building bombs, which cost only a few hundred pounds to put together. It finds nothing to suggest an al-Qa'ida "fixer", thought to have come from Pakistan, masterminded the attacks, and it uncovers no proof that a fifth bomber was involved in the conspiracy.
The report acknowledges that the bombers - Mohammad Sidique Khan, Hasib Hussain, Germaine Lindsay and Shahzad Tanweer - could have been influenced by Khan's visits to Pakistan. But it paints a picture of a homegrown operation by young men motivated by anger over Western policy towards the Middle East and by the promise of immortality.
A videotape released shortly after the attacks featured Khan and Osama bin Laden's deputy, Ayman al-Zawahiri. However, the Home Office report suggests it was edited at a later date to give the false impression of involvement by al-Qa'ida.
It will not address the question of why the security services failed to keep track of Khan after he became known to them as a possible risk before the attacks. That apparent failure could be highlighted by the Commons' Intelligence and Security Committee, which is also investigating the attacks.
The Independent has learnt that ministers privately accept they must do more to reassure the public over Britain's vulnerability to suicide attacks.
They are therefore expected, before the first anniversary of the attack, to set out in detail how defences have been tightened, pointing to the recruitment of extra officers to MI5 and MI6 and success in preventing a number of terrorist strikes.
Patrick Mercer, the Conservative spokesman on homeland security, said he found it "very hard to believe" the conclusion that the bombings had no link to al-Qa'ida.
"A narrative from the Government is going to come from the same sources that provided us with the dodgy dossier over Iraq. This is why it is so important that we have an independent inquiry and not just another government whitewash," Mr Mercer said.
A Home Office spokesman said last night: "We won't comment on a leaked document and we won't pre-empt the official account of 7 July, which we intend to publish shortly."