The campaign office of Labour chairwoman Hazel Blears has been evacuated after part of the central London building collapsed.
The top floors of the building collapsed
The top floor of the building, in which Ms Blears has offices on two floors, collapsed on Tuesday.
Six volunteers for her deputy leadership bid were in the building at the time, but none were hurt.
It is not yet known what caused the building to collapse but it is thought it was being renovated.
Emergency services were called to the building, in Dean Farrar Street near New Scotland Yard, Westminster, at 1600 BST.
The whole top floor of the building had been blown away and eyewitnesses say they heard an explosion.
Sean Newman, one of the volunteers for Ms Blears' campaign, said: "Some of us who were upstairs felt the building shake.
"Then we heard the alarm go off and left our stuff to go downstairs. It was a calm evacuation, but they would not let us go back in."
A spokesman for Ms Blears said was a business centre, where office space was hired by different ventures.
"It is usually packed with people, but our activists did not see anyone injured," he said.
Among others in the building was Justin Linger, who told BBC News 24: "It was more than a noise - it was just a feeling of the building actually physically moving. It was quite disconcerting."
But he added: "To be fair, most people left the building in a fairly orderly fashion. There was no panic particularly."
One person is thought to have been hurt, and has been treated in hospital for minor injuries.
Ms Blears is one of six contenders for John Prescott's job, when he steps down with Prime Minister Tony Blair at the end of June.
She and several other MPs spent several hours in the offices during the morning, phoning potential supporters.