Four builders were injured today in a "non-terrorist" explosion at a police station.
The building in Croydon, south London, was evacuated after the blast, which left one contractor in a serious condition, a Metropolitan Police spokesman said.
Health and safety investigators have been called in to investigate the incident, which happened at 11.25am.
The explosion caused a fire in the basement area of the police station, a spokesman said.
"Inquiries are under way to establish the cause of the incident," he said. "This incident is not believed to be terrorist- related at this early stage."
Both London Fire Brigade and the ambulance service were at the scene.
The police spokesman added: "It is believed four people have received injuries.
"One is believed to be in a serious condition, one is believed to be in a stable condition and two are believed to have suffered from smoke inhalation.
"Croydon police station has been evacuated and cordons remain in place."
He said the borough remains "fully operational" for police, adding: "There has not been any disruption to service."
The force was unable to specify what building work was taking place involving the contractors.
A London Fire Brigade spokesman said six engines and 30 firefighters were sent to the scene in Park Lane.
"Fire crews from Woodside, Croydon, Norbury, Addington, Tooting, Wallington and Brixton fire stations are attending the scene," he said. "The cause of the fire is not known at this stage."
Apart from the four builders, three other people were also injured, police said. A total of five victims required hospital treatment.
"Of those injured, one was initially believed to be in a serious condition, and another was believed to be in stable," a statement from the force said.
"Two are believed to have suffered from smoke inhalation. We await advice on those others."