He had been working on a building site close to the venue of the Fashion East show on Gray’s Inn Road in central London – but it is not yet clear why he was walking along the roof.
The man, thought to be in his 30s, fell from a height of around 10 metres (30ft) into a backstage area after a corrugated glass roof collapsed, according to witnesses.
It is thought that he landed on his back and his head struck scaffolding as he fell.
Topshop, which sponsors the Fashion East event, was criticised after it continued the show in spite of the incident, which occurred about half-way through the programme.
A spokesman for the company said the decision was made because the builder was immediately attended to by both medical staff and production security.
London Ambulance Service said a doctor accompanied the man to Royal London Hospital, where he was dealt with “as a priority”.
A spokesman for the service said they sent both an ambulance team and an air ambulance to the scene and treated a man his 30s for back injuries. He was then taken to hospital by road ambulance.
The accident happened at the Topshop Show Space, a huge garage near King’s Cross station previously used for classic cars. The building is expected to be demolished and replaced by luxury apartments in the near future.
Fashion East was launched to help young designers in the early stages of their career. It is being funded by Topshop, Topman and the Greater London Authority.
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