An explosion on a District line tube service was caused by an improvised explosive device, police have confirmed.

The Metropolitan Police, which earlier confirmed the incident was being treated as a terrorism , is now being supported in its ongoing investigation of the incident by MI5 intelligence experts.

Members of the London Ambulance Service , London Fire Brigade and British Transport Police are attending the incident, with reports the military are also providing assistance.

Police were called around 8.20am following reports of a fire on the train .

Photos from the scene show a bag on fire with wires coming out, after emergency services were called to the west London station during the Friday (September 15) morning rushhour.

Eyewitnesses reported seeing "a bag, a flash and a bang."

Assistant commissioner Mark Rowley said: "We now assess that this was a detonation of an improvised explosive device.

"The scene currently remains cordoned off and the investigation continues.

"In terms of the police response, our well-practised command structures have come into shape.

"There are many urgent inquiries ongoing with hundreds of detectives involved looking at CCTV, forensic work and speaking to witnesses."

"This investigation is supported by our colleagues at MI5 and bringing their intelligence experts in on the case."

For live updates on this incident, head to our live blog.

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