British Prime Minister Tony Blair has arrived back at the Group of Eight summit in Scotland after returning to London this afternoon for crisis talks following four blasts that killed 37 people.

Mr Blair pledged earlier this evening that Britain would not be intimidated by the terrorist bombings which killed at least 37 people in London today.

He promised intense police and security service action to bring the bombers to justice and he repeated his profound condolences to the families of the victims.

Mr Blair had returned to London this afternoon to be briefed on the attacks.

He promised to return to Gleneagles later today to rejoin the G8 meeting, which is due to end on Friday.

Earlier he said the people of Britain were more determined to defend the values they hold dear than the terrorists were to destroy them.

Queen Elizabeth II said she was 'deeply shocked' by the wave of terrorist attacks.

Clarke in emergency Commons statement

British Home Secretary Charles Clarke confirmed at lunchtime that at least four explosions had taken place in terrorist attacks across London.

In an emergency statement, Mr Clarke told the Commons he did not know 'who or which organisations' were responsible for these 'criminal and appalling acts'.

He said the London Underground was closed and would remain so and he strongly advised people not to travel into central London.

The first explosion was on a tube train between Aldgate East and Liverpool Street, the second on a bus in Woburn Place, the third on a tube train between Russell Square and King's Cross and the fourth on a tube train at Edgware Road station.

Attacks ‘cowardly’: Livingstone

The Mayor of London, Ken Livingstone, said the attacks were 'cowardly terrorist attacks' and that the people of London had responded ‘calmly and courageously’.

'I commend the professionalism, courage of the Metropolitan Police, Transport for London staff, the Ambulance, Fire and other emergency services. On behalf of all Londoners, I want to express my deepest sympathies to the families and friends of those who have been killed and to all of those who have been injured,' said the mayor.

He continued: 'Innocent people going from all of London's communities have been targeted by this indiscriminate attack. I urge Londoners from all of this city's diverse communities and faiths to support one another and stand together against terrorism.'