Despite calls for an inquiry into Scotland Yard’s tactics after the killing of an innocent Brazilian, the Metropolitan Police Commissioner, Sir Ian Blair, insisted that shoot-to-kill orders will stay in force.

Charles Clarke, the Home Secretary, has postponed his family holiday to take charge of the fraught manhunt. Three days after Scotland Yard issued CCTV photographs of the fugitives, none has been arrested.

But Sir Ian praised his officers, saying that “they are playing out of their socks”.

It emerged last night that a man was arrested on the street in Tulse Hill, South London, on Saturday but, like two others held on Friday, he was not thought to be one of the bombers. The three are being questioned at Paddington Green police station.

Police sources also disclosed that a bag found at the weekend in a park in Wormwood Scrubs, West London, contained the same type of explosives that were used in Thursday’s attacks. They believe that a fifth bomber may have abandoned his mission for unknown reasons. Detectives do not know how much explosive material the cell still has.

Security forces fear that the four gang members — all believed to be London-based and of East African origin — will strike again before they are found or the explosives degrade.

Sir Ian said that his force took full responsibility for the death of Jean Charles de Menezes, a 27-year-old electrician, but said that there would be no change of orders to his 2,000 armed officers. Politicians and police chiefs believe that the risk of more mass killings is graver than another blunder.

As Sir Ian apologised, the Brazilian Government demanded an explanation and Mr Menezes’s family and friends protested outside Scotland Yard.

Police insisted last night that the block of flats in Tulse Hill where Mr Menezes lived, was linked to one of the bombers by evidence found in one of the unexploded rucksack bombs.

Sir Ian said that the Brazilian had been seen leaving the address but admitted that he did not how many people shared the building.

Mr Clarke will attend a Downing Street meeting tomorrow called by Tony Blair to brief opposition leaders on his meetings with Muslim leaders, police and intelligence services.

Detectives have established links between the fugitives and the four July 7 terrorists. The two groups are believed to have met at an adventure holiday centre in North Wales. There are also telephone links between the two cells.

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