The BBC's Lucy Atherton: The caller may have been captured on video

A man, saying he was from the extreme right-wing group Combat 18, rang police on Monday morning to claim responsibility for the attack at Brixton's Electric Avenue market.

The bomb injured 48 people, including a 23-month-old boy who was left with a four-inch nail embedded in his brain.

Forensic scientists search for clues

The 999 call was made at 0606 BST from a telephone box in Well Hall Road, Eltham, close to where black teenager, Stephen Lawrence, was murdered in April 1993.

'Keeping an open mind'

But Scotland Yard detectives say they are keeping an open mind about Saturday's attack. They say the phone call may have been a hoax.

A spokesman said that, unlike the IRA or other groups with bomb-making experience, Combat 18 did not have a system of recognised codewords.

Nick Knowles of Searchlight: It's a cowardly act

He said no such codeword was used by the caller and the force was unable to confirm whether or not Combat 18 were responsible for the bomb.

Steve Silver, co-editor of the anti-fascist magazine Searchlight, said: "The unpleasantness of it all is certainly the sort of thing you would expect from Combat 18 and if they were going to carry out such an attack then Brixton is the sort of place they would do it.

'Unlikely to be them'

Aiah Menjor describes the moment the bomb went off

"But they have not carried out this kind of bombing before which makes me feel it is unlikely to be them, unless they are trying something new."

The police are anxious to speak to the person who made the phone call and have appealed for help from anyone who may have been in the vicinity of Well Hall Road around 0606 BST on Monday.

The door of a delivery van was blown off in the blast

Police are sifting through recordings from nearby security cameras to see if the bomber was caught on video.

Officers have been interviewing scores of witnesses and are trying to eliminate from their inquiries a "blond man" seen outside the Iceland supermarket shortly before the bomb exploded.

The bomb, placed inside a black Head sports holdall, was packed with nails of between four- and ten-inches in length.

Police are anxious to trace anyone who may have innocently sold a bulk order of nails in the days or weeks running up to the bombing.

Horrifying injuries

Of the 48 people injured in the blast, 13 are still in hospital, with four giving doctors cause for concern.

But the youngest victim of the bombing is making a good recovery after undergoing surgery to remove a four-inch nail from his brain at Great Ormond Street Hospital.