By late Friday afternoon police and emergency services had recovered 49 bodies, including all 13 from the bus bombing.

An unknown number of bodies still are in the underground railway system, with rescue efforts being hampered by the fear that the tunnels may be unsafe as a result of the explosions, London's police said.

"We now know that there are more than 50 fatalities," Metropolitan Police Commissioner Ian Blair told a news conference on Friday.

"There is great difficulty in deciding or determining how many fatalities, because two of the scenes are very difficult in terms of recovery," he said.

"One is the bus, because of the nature of the explosion and more acutely the tube train at Russell Square (subway station in central London) continues to hold a number of bodies.

"We don't know how many are there."

Flowers are left  in memory
of the bomb victims

A total of 350 people were treated for light injuries at the scene. Another 350 had been taken to hospital, of whom 100 were kept in overnight. Twenty-two people were still in serious and critical conditions, he said.

One person had died in hospital during the night.

Blair said the fact that an unknown number of bodies had not yet been retrieved should not be taken as a suggestion that the death toll could climb above 100.

Tunnels unsafe

"It's yet to be the case for us to get near the carriage. There is a risk of the tunnel being unsafe. I ask everyone's patience as we progress this matter," Assistant Commissioner Andy Hayman told the same news conference.

"It would be wholly unwise and could inhibit a successful prosecution if we rushed this stage," he said.

Hayman said each bomb likely contained less than 4.5kg of explosives.

In their attack on the subway system, the attackers likely placed the bombs on floor of the train carriages, he said.