Foreign Secretary David Milibandhas condemned a "despicable and indiscriminate" car bomb attack which killed 91 people and injured more than 200 in a crowded market in the Pakistani city of Peshawar.
The blast happened just hours after US secretary of state Hillary Clinton arrived in the country to show American support for the Pakistan authorities' campaign against Islamist militants.
Mr Miliband offered his and the UK's condolences to the victims and their families.
The Foreign Secretary said: "We utterly condemn this despicable and indiscriminate attack against the Pakistani people. Our thoughts and condolences go to the victims and their families. The UK will continue to support Pakistan in its efforts to counter the threat it faces from violent extremism."
The blast set scores of shops on fire and sent a cloud of grey smoke over the city. TV footage showed wounded people sitting amid the debris as people grabbed at the wreckage, trying to pull out survivors. One two-storey building collapsed as firefighters doused it with water.
Ms Clinton, on her first visit to Pakistan as secretary of state, was three hours' drive away in the capital of Islamabad when the blast took place.
Speaking to reporters on her plane, she praised the army's new anti-Taliban offensive in South Waziristan and promised a new era in relations between Pakistan and the United States.
The explosion was in a neighbourhood home to many Shiite Muslims, who have often been targeted by Taliban and al Qaida allied Sunni extremists. It hit a market reserved for families. Many of the dead were believed to be women.
No group immediately claimed responsibility, but Peshawar has been the target of three of 10 major bombings or raids blamed on Islamist militants this month. The attacks have killed more than 250 people.
Tuesday's explosion was the latest in a series of bloody attacks this month apparently designed to undermine public support for the Pakistani army's offensive against Taliban and al Qaida strongholds close to the Afghan border.