Investigators are increasingly convinced that only one bomber — who killed 13 people in the explosion on a double-decker bus — died in the blasts.

The others are thought to have left their bombs — consisting of less than 10lb of high explosive hidden in rucksacks and fitted with timed fuses — on the floors of three Tube trains before escaping.

New information emerged last night on the timing of the explosions on the Undergroud. Police said that they now believed that the bombs went off within six minutes of each other, the first at Edgware Road station at 8.50am. This was originally logged as a person under a train, but by 9.17 police had realised that it was a bomb.

The second blast, between Aldgate and Liverpool Street on the Circle Line, came at 8.51, with the third, on the Piccadilly Line train between King’s Cross and Russell Square, at 8.56. The bus explosion in Tavistock Square came at 9.47.

One high-level source said that investigators were assuming that “the people who did this are still out there. They could do it again”.

A second attack would fit the pattern of recent al-Qaeda activity in Europe. In November 2003, the HSBC bank and the British Consulate in Istanbul were attacked five days after two synagogues in the city had been bombed.

The London bombs are strikingly similar to the wave of blasts which killed 191 people on commuter trains in Madrid in March last year. Less than a month later, a bomb attack on the high-speed rail line to Seville was foiled.

Scotland Yard’s Anti-Terrorist Branch is in contact with its Spanish counterparts and police and security services around the world to try to discover any intelligence that can identify the bombers.

A main concern is that they are dealing with “clean skins”, possibly British-born terrorists who have not crossed the intelligence radar before. Whoever the killers are, they have access to high explosives and bomb-making expertise.

A police source told The Times: “Our main fear is that this group is out there still sitting on a cache of high explosives knowing that their bomb designs worked.

“We know from the two most recent atrocities in Europe that those groups always intended to make two attacks. Instead of going for perfect synchronicity in one spectacular, they have tried to hit the same target twice.”

The confirmed death toll in Thursday morning’s four blasts has risen to 49 but police say that it could reach 70. Nobody knows how many bodies are inside a wrecked carriage in a deep, badly damaged tunnel between King’s Cross and Russell Square stations.

Some of the 86 people still in hospital were visited by the Queen, the Prince of Wales and the Duchess of Cornwall.

The Queen, visiting the Royal London Hospital in Whitechapel, told staff: “Atrocities such as these simply reinforce our sense of community, our humanity, our trust in the rule of law. That is the clear message from us all.”

Services on parts of the Underground will not be back to normal for several weeks. Transport for London said that Circle and Hammersmith & City line services, which were suspended yesterday, will resume in days but that the central section of the Piccadilly Line would not be back in action for some weeks.

The bombings overshadowed yesterday’s deals at the G8 summit in Gleneagles on climate change, alleviating debt among developing countries and aid to Africa. Flanked by other world leaders, Tony Blair contrasted its achievements with the “despair and hatred that terrorism sought to put in people’s hearts”.

He highlighted an agreement on $3 billion for the Palestinian Authority after the Israeli disengagement from Gaza, saying that it showed how civilised nations could help “two peoples and two religions live side by side in peace”.

After returning from the summit, President Bush went to the British Embassy in Washington to sign a book of condolences for the victims.

Ministers emphasised yesterday that they would not seek to make political capital out of the bombings. Charles Clarke, the Home Secretary, said that they would not have been prevented by identity cards. He has called a meeting of his European counterparts in response to the attacks, in Brussels on Wednesday.

Mr Clarke also suggested that he may agree to a Tory request for an inquiry into any failings by the security services.

Senior Whitehall sources believe that there were no warning signs because the terrorists had planned an attack to coincide with the G8 summit a long time ago. “We did not hear any chatter because there may have been none,” one said.

The police inquiry, the biggest manhunt in Scotland Yard’s history, is focused on the recovery of CCTV film of the bombers boarding or leaving trains. But forensic science analysis is being severely hampered by conditions in the tunnels.

Sir Ian Blair, the Metropolitan Police Commissioner, said that the bombings would effect the whole of the UK. “This is a national issue, not just for London,” he said.

Explore UK News

• Crime News

• Education News

• Health News

• Science News

• Scotland News

Times Recommends

• DIY video helps the WI get to grips with sex
• Artists urge nation to save Titians
• Go home, Iraqi leader tells Britain