British Prime Minister David Cameron has said those killed in the 2005 London bombings would ‘never be forgotten’, as low-key ceremonies marked the fifth anniversary of the attacks.

The 7 July 2005 suicide bombings on three London Underground trains and a bus left 52 people dead.

No official commemoration events took place, although wreaths were laid on behalf of Mr Cameron and London Mayor Boris Johnson at the official memorial to the victims in London's Hyde Park.

Many survivors and relatives of victims gathered for private ceremonies at the sites of the four explosions.

‘People in our country will remember where they were and what they were doing when that dreadful news came through,’ Mr Cameron said in parliament.

‘Our hearts should go out to the families and friends of those who died. They will never be forgotten.

‘Our thoughts are also with those who were injured, physically and mentally, by the dreadful events of that day.

‘It was a dreadful day but it is also a day that will remain, I believe, a symbol of the enduring bravery of the British people.’

The attacks were carried out by British Muslims as London was still celebrating the decision announced the previous day to award the city the 2012 Olympics.

They were followed two weeks later by an attempt to replicate the bombings but the homemade explosives failed to detonate.

Intelligence services have warned that Britain remains at high risk from Islamic extremists.

The current terrorism threat level is ‘severe’, meaning that a terrorist attack is considered highly likely.