The BBC's Stephen Cape: "Forces around Britain will be on alert this weekend"

Thirty nine people were injured when the device exploded outside a supermarket on 17 April.

On Thursday police showed CCTV footage and stills of a white man in his 20s, approximately 5ft 6in to 5ft 8in tall, wearing a white baseball cap and a zipper jacket.

Police video of the man they want to question

In the video he is shown walking along a crowded street in Brixton, south London, an hour-and-a-half before the bomb detonated, injuring 37 people.

Assistant Commissioner David Veness said detectives were "satisfied" he may have been in possession of the bag containing the bomb.

Police poster of the Brixton suspect

"We have a range of material ... that leads us to the unequivocal conclusion that the investigation focuses on this man's face," he said.

"It's our number one priority to identify this man."

Police believe he may have been checking the area before returning to plant the bomb.

The BBC's Charles Amponsah: "Somebody knows this man"

No accomplices were identified in the footage taken around Brixton Road and Electric Avenue near the Iceland store in Brixton where the bomb exploded.

Mr Veness said: "This one individual may be a loner acting for his own perverted reasons."

But he refused to rule out the possibility that he could be part of a "significant grouping".

Six were injured in the attack in Brick Lane

The images, which were sent to the United States to be enhanced, did not reveal anyone on the police database of known right-wing extremists.

It is the most significant development so far in the police investigation into nail bombs that have targeted busy shopping areas in Brixton and Brick Lane in London's East End.

An extreme right-wing group, Combat 18, said it carried out the Brick Lane attack. Four racist groups claimed responsibility for the Brixton bombing, including a group calling itself the White Wolves.

The BBC's Stephen Cape: "Just a face in the crowd?"

Nobody died in either of the bombings, but dozens of people were injured, including a toddler who had to have a four-inch nail removed from his skull.

Scotland Yard is helping to co-ordinate what it calls "defensive responses" across the country following the bombings in London.

It has drafted extra detectives into the anti-terrorist branch and stepped up the hunt for far-right extremists.