The accident occurred in the North East corner of the stadium where a new club shop and 400 extra seats are being built between the Vicarage Road and Sir Elton John stands.

The Enquirer understands that soffit formwork installed ahead of a concrete pour collapsed at the site.

The injured worker was taken to the Royal London Hospital by air ambulance.

One local contractor said: “It’s awful what has happened on the site.

“The formwork can’t have been designed properly to just give way like that.”

Cliff Webb, watch commander from Watford Fire Station, told the Watford Observer: “We were called to a trapped person. He had fallen from height.

“He was in a difficult position to get to. We made sure it was safe for the paramedics and then we tried to get some spine boards through as quickly as possible.

“An air ambulance landed on the pitch and we carried him through the building works then on to the pitch.

“He’d fallen from a part of the new stand.

“He was conscious and breathing and had head and leg injuries and a possible pelvic fracture.”

Site hoardings on the collapsed part of the ground promote a firm called ECS Group. The company has been contacted for a comment.