A train that hit another train in a tunnel was travelling at 80 miles per hour just two minutes earlier, accident investigators said.

A northbound London Midland train was travelling through a tunnel on the West Coast Main Line in Watford when it struck a southbound train derailed by a landslip.

The Rail Accident Investigation Branch (RAIB) said the driver of the southbound train used an emergency facility on the in-cab radio to alert other trains about the derailment.

SEE MORE: Hundreds rescued after train derails at Hunton Bridge.

The driver of the northbound train received the message and applied the brake, slowing the train from 80 miles per hour to 32 miles per hour before the collision occurred around two minutes after the first train hit the landslip.

Two passengers suffered minor injuries in the incident shortly before 7am on September 16.

The two trains, which were carrying more than 150 people, were both significantly damaged, the RAIB said.

Announcing that a full investigation is to be carried out, the RAIB said the landslip material which caused the derailment had slid from the steep-sided, deep cutting on the tunnel approach after very heavy rainfall in the two hours before the accident.

The sides of the cutting were being strengthened at the time of the accident, but this work had not yet reached the location where the landslip occurred.

The investigation will consider why the landslip occurred, the response to the heavy rainfall and what happened between the derailment and the subsequent collision.