Croydon blast injures four builders © REUTERS / Russell Boyce A builder is in a serious condition following an explosion at a south London police station. Three other workers are also believed to be injured. The explosion at 11.25am caused a fire in the basement area of the police station in Croydon. A Metropolitan Police spokesman said the incident was a "non-terrorist" explosion. He said: "Inquiries are under way to establish the cause of the incident. 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. He said the borough remains "fully operational" for police, adding: "There has not been any disruption to service." Both London Fire Brigade and the ambulance service were at the scene and the building has been evacuated. A London Fire Brigade spokesman said six engines and thirty firefighters were sent to the explosion on 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. The force was unable to specify what building work was taking place involving the contractors.