Croydon's police headquarters was evacuated after an explosion in the basement left seven people injured.

Fifty firefighters, three ambulances and the air ambulance helicopter descended on the Croydon fly-over roundabout in the town centre after an explosion in the station’s basement at 11.25am Tuesday March 20, brought the area to a standstill.

Four contractors from Vinci Construction UK, scheduled to be refurbishing a decommissioned fire range in the basement, were caught up in the blast, with one airlifted to a specialist unit at the Broomfield Hospital, Chelmsford suffering from severe burns.

A second man was taken to St George’s Hospital with burns to his face and arms, a third contractor sustained minor burns and a fourth smoke inhalation, while two staff and a police officer were also treated for smoke inhalation.

A spokesman for the police said the firing range, used by forces across London, had been decommissioned during refurbishment, which saw the station only reopened to the public on Sunday March 18.

Any ammunition stored in the basement is believed to have been removed.

A spokesman for Vinci said: "Vinci Construction UK is gravely concerned about the welfare and safety of the construction workers and others affected by this incident and has been working with all emergency services throughout the day to secure the scene of the incident. We are working with the HSE in their ongoing investigation to understand the cause of the explosion."

Borough Commander Chief Superintendent Dave Musker said: "Seven people have been injured and five have been taken to hospital. While two are thought to be seriously injured.

"We want to reassure the public we continue to provide fully operational services to the community of Croydon.

"Our thoughts and wishes are with those injured."

He confirmed the incident was not terrorist related.

London Fire Brigade Station Manager Tej Ajimal, at the scene, said: "Crews worked hard to bring the fire under control and managed to contain it to the basement area and stop it from spreading to the rest of the police station."

Chf Supt Musker said: "Health and Safety Executive are investigating the circumstances of the incident and are being assisted by CID officers and London Fire Brigade.

Fire fighters from 10 crews fought for two hours to prevent the fire spreading out of the basement.

Police operations were moved to South Norwood Station, alongside 14 who were in police custody.

Essential staff moved back into the station just after 3pm and the station was expected to be fully re-opened overnight.