Four suspects who fled after attempted bombings last week in London are probably still in Britain and the police anxiously await public sightings of them, London's police chief Sir Ian Blair said yesterday.

Blair said he was confident of a breakthrough in the "incredibly fast-moving" investigation - the biggest in British history - into the bombings on London's public transport system on July 7 and attempted attacks on Thursday.

"We are still anxious for any sightings of the four individuals and there are inquiries all over London and indeed in the rest of Britain," Blair told Sky News.

Police said they arrested a man in south London last night under anti-terrorism laws, a day after two men were detained separately in the nearby Stockwell neighbourhood following armed raids. An unconfirmed media report said one of the detainees was the bomber who targeted the No 26 bus on Thursday.

Weekend papers were plastered with security camera pictures of the men, taken at three Underground stations and a bus, prompting hundreds of phone calls and e-mails to police hotlines.

Asked if he believed the would-be bombers were still in Britain, Blair replied: "We have no reason to believe they're not."

Blair said police had also found a common thread between the July 7 bombings and Thursday's attempts, because both targeted Underground trains and buses and because of similarities in the explosives.

"The equipment in the (Thursday's) bombs had all the elements it should have and it didn't work," he said, adding police were still studying the explosives.

"We haven't proof there are linked but clearly there is a pattern," he said.

Apart from sparing the city a likely repeat of the carnage on July 7, when attacks killed all four bombers and 52 other people, the botched blasts were a massive help for police on two fronts.

Firstly, the abandoned bombs appeared likely to be packed with forensic evidence and other vital clues.

Also, while police are desperate to track down the four bombers before they try to strike again, they are hopeful the men's movements while on the run could betray a wider support network.

Perhaps even more crucially, reports said officers police found documents in one of the rucksacks from the failed bombings that led them to believe there might have been a link between the two sets of attackers.

According to The Observer newspaper, some of the suspected bombers from Thursday are believed to have been on a whitewater rafting trip in Wales attended by two of the July 7 bombers, Mohammad Sidique Khan and Shahzad Tanweer.

The trip could even have been a "bonding" exercise for all concerned, meaning the attacks were planned jointly, the paper said. "One line of inquiry being pursued is whether a cleric was responsible for organising the trip to bring the two cells together," it added.

All four of the July 7 suicide bombers were named by police as being British Muslims, three of Pakistani origin and one born in Jamaica.

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