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Prime Minister David Cameron held the hand of a woman who had collapsed until medical assistance arrived, the London Ambulance Service has said.

Mr Cameron was on the campaign trail with London Mayor Boris Johnson in Harrow when they reportedly chanced upon the woman in the street.

A paramedic who attended the scene, Dominic Stark, said: "I took over caring for the patient and they left a few minutes later."

The woman was then taken to hospital.

The BBC's Chris Mason said he had been told the ambulance staff "looked like they had swallowed a very large fly" when they found the PM and London mayor tending to the distressed woman.

He said he understood the two politicians had comforted the woman - who was conscious at the time - for a couple of minutes before the emergency services arrived on the scene.

"The prime minister and Boris Johnson had pulled over when they noticed a woman collapsed on the street," Mr Stark explained.

"When I arrived a few minutes later, I found the PM holding the woman's hand. I took over caring for the patient and they left a few minutes later."

The woman at the centre of incident was recovering, he added.