Security sources said it was likely at least three of the men, said to be of Pakistani descent, are dead, after belongings were found at the scenes.

The details emerged as explosives were found in Leeds and Luton after a series of raids. One man has been arrested.

The BBC's Frank Gardner said an expert may have offered the bombers guidance.

It appears our youth have been involved in last week's horrific bombings - nothing in Islam can ever justify the evil actions of the bombers

Sir Iqbal Sacranie, Muslim Council of Britain

Analysis: Worst fears true Q&A: Hunt for bombers Viewpoint: Bombers ready to die

The security correspondent said the suspected bombers - one of whom is thought to have been as young as 19 - may have been helped by someone who would have left the country before the bombs went off.

Police revealed details of the breakthrough in their investigation into the attacks, which killed at least 52 people, on Tuesday.

It emerged that relatives of one of the men had reported him missing last Thursday morning.

On Monday night, police had viewed CCTV footage of four suspects together at London King's Cross last Thursday.

They all had rucksacks and were seen just 20 minutes before the three Tube bombs started going off at 0851 BST. A bus bomb went off in Tavistock Square at 0947 BST.

Three of the men had travelled to Luton from Leeds by train, before catching a Thameslink train to London. They had been joined at Luton by a fourth man who had driven to the Bedfordshire town.

'Shock and horror'

Tuesday's police raids, which began at 0630 BST, centred on two properties in Dewsbury, West Yorkshire, and four in Leeds.

WEST YORKSHIRE RAIDS 1. 0630 BST: Two houses raided in Beeston 2. Two houses raided at same time in Thornhill, Dewsbury 3. One House raided in Holbeck 4. 1320 BST: Controlled explosion in Burley

Suspects: Key facts Cars linked to attacks

Police said they had arrested a relative of one of the four suspects in Yorkshire and taken them to London for questioning.

Explosives were also found in a car at Luton railway station, where experts have carried out seven controlled explosions, with three more expected to follow.

A second car believed to be linked to the attacks was also found at the station and towed to Leighton Buzzard, 10 miles (16km) west of Luton, for further examination.

Sir Iqbal Sacranie, of the Muslim Council of Britain, said it had received news of the suspects with "anguish, shock and horror".

He said: "It appears our youth have been involved in last week's horrific bombings against innocent people.

"While the police investigation continues we reiterate our absolute commitment and resolve to helping the police bring to justice all involved in this crime of mass murder. Nothing in Islam can ever justify the evil actions of the bombers."

Metropolitan Police Commissioner Sir Ian Blair said the searches, carried out under the Anti-Terrorism Act, were intelligence-led and "directly connected" to last week's attacks.

Head of the Metropolitan Police anti-terrorist branch Peter Clarke said documents identifying three of the men were found near three blast sites.

But there is no identity for the fourth bomber and police do not know if his remains are at the King's Cross blast site or if he has fled.

Three of the four men were from the West Yorkshire area, said Mr Clarke.

His colleague, assistant commissioner Andy Hayman, said: "I want to conclude by making it absolutely clear that no-one should be in any doubt the work last Thursday is that of extremists and criminals.

"No-one should smear or stigmatise any community with these acts."

Following developments on Tuesday he described the investigation as "complex and intensive" and "moving at great speed".

BOMB MANHUNT London: Forensic work ongoing at blast sites Leeds area: Six houses searched, controlled explosion at one, one arrest made Luton: Controlled explosion after car find near station

List of bomb victims Leeds neighbours' shock

Police said there was forensic evidence that one of the bombers died in the Aldgate Tube explosion.

Property belonging to one of the suspects from West Yorkshire, who was reported missing by his family just after 1000 BST on Thursday, was also found on the devastated bus.

A third man's property was found at both the Aldgate and Edgware Road blasts.

Mr Clarke said: "We are trying to establish their movements in the run up to last week's attacks and specifically to establish if they all died in the explosions."

Eleven victims of the blasts have now been formally identified.

Eight inquests will be opened on Wednesday, including those of Londoners Jamie Gordon, 30, and Phillip Russell, 29.

Police are asking for anyone with information on the bombs to contact their anti-terrorist hotline on 0800 789 321.