Terrorism was swiftly ruled out as a cause.
Four Vinci employees were among seven injured. They had been working on refurbishing what used to be a Scotland Yard firing range in the basement of the building when the explosion occurred at 11.25am.
A police spokesman said the firing range had been decommissioned for the refurbishment, which was scheduled to complete in May. It is believed that no weapons or ammunitions had been left in the basement.
A Vinci spokeswoman 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."
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