LONDON (AFP) -- The failure of British spies to prevent last year's London bombings was largely down to a lack of resources, two reports into the attack are expected to conclude this week, newspapers said Sunday.

Among the findings in the reports, one by a committee of MPs and another by civil servants, is that one of the bombers had links with a terrorist cell.

Turning to what can be learnt from the attack, the lawmakers' report will propose that Britain uses a color-coded warning system to alert the public to a terrorist threat, similar to the one used in the United States.

The official accounts of the bombings, in which 56 people died, including four apparent suicide attackers, are due to be published on Thursday.

Mohammad Sidique Khan, 30, Shehzad Tanweer, 22, Hasib Hussain, 18, and Germaine Lindsay, 19, are thought to have blown up three London Underground trains and a double-decker bus during the morning rush hour on July 7, 2005 by detonating bombs packed into rucksacks.

The News of the World tabloid said the report by the parliamentary Intelligence and Security Committee concluded that British spies failed to infiltrate radical Muslim groups because of a severe "resource problem".

The committee will reveal that at one point a mere 20 agents were trying to follow the movements of 400 terror suspects.

Surveillance of Khan, the presumed ringleader of the London attacks, was lifted because of a lack of manpower, the Sunday tabloid said. -------British M15

The report will also note that MI5, Britain's domestic intelligence agency, has too many white, middle-class agents who were unable to go undercover in Muslim communities, according to The News of the World.

And it warns of future attacks unless MI5 is overhauled.

In a potentially damaging revelation, the committee found a direct link between Khan and a terrorist cell that had been under surveillance, according to The Observer newspaper.

It had previously been believed that Khan was only connected to the cell through a third party, it said. "That he had direct links to the group under surveillance raises questions over why he was not placed under closer supervision," the weekly added.

The Observer revealed last month that the government's report on the bombings concluded they were planned on a low budget from information off the Internet and Al-Qaeda played no part.

The Sunday Times focused on a proposal by the intelligence committee to introduce color-coded terror alerts in Britain.

At present, MI5 keeps its threat assessment private but the ISC suggests adopting a more open system like the one used in the United States.

MI5 mistakenly reduced its threat assessment one month before the July 7 attacks and was apparently planning to lower it again on July 21 -- the day four men carried out a failed attempt to repeat the London bombings.