The Channel Tunnel is finally to have a sprinkler system fitted after the third fire in 12 years forced Eurotunnel to admit that it had underestimated the risk of disaster deep under the sea bed.

Burning trains will stop at “extinguishing stations” that will be built at intervals along the 31-mile (50km) twin tunnels. Heat detectors will locate the fire and high-pressure jets of water or foam will be aimed automatically at the source as soon as the last passenger has escaped into the service tunnel.

Eurotunnel, the French-dominated company that operates the tunnels, is being forced to act after a lorry caught fire in September and caused a 1000C (1830F) inferno that melted cables and destroyed the tunnel’s concrete lining. Millions of passengers and lorries have suffered delays and cancellations for the past five months because all trains have been diverted to the other tunnel. Repairs are costing more than £50 million and normal train services will not resume until February 23.

It was the third time that a lorry travelling on an open wagon had caught fire, the previous incidents being in 1996 and 2006. Car shuttle trains have carriages that can be sealed and filled with a suppressing gas in the event of a fire

Eurotunnel spent years debating, then testing, sprinkler systems after the first lorry fire, which closed one tunnel for six months. But it refused to pay for a system to be installed and claimed that the risk of a similar fire was remote.The company has now admitted its mistake and is drawing up plans for an advanced sprinkler system, similar to one that will be installed on the new rail tunnel under the Alps connecting Lyon and Turin. The system will first have to be approved by the Intergovernmental Commission, which oversees tunnel safety. It is unlikely to be installed before the end of next year.

Bruno Bouthors, Eurotunnel’s safety director, told The Times: “People might ask why it wasn’t done before, but the fire in 1996 was thought to be exceptional and the one in 2006 was small. The latest fire is showing we need to prepare for the worst case.”

He said that the system would prevent fires from spreading out of control, allowing time for the firefighters to get to the scene via the service tunnel. In the fires in 1996 and in September, firefighters arrived too late to prevent severe damage.

Mr Bouthors said that the extinguishing stations would be located so that a train could arrive at one within 15 minutes of a fire being detected. He admitted that a fire-suppression system had been considered before the tunnel opened in 1994. He said that it was easy to be critical in hindsight, but at the time the risk of a lorry fire was thought to be very low.

Norman Baker, the Liberal Democrat transport spokesman, said that Eurotunnel was acting after years of “putting cost before safety” but it was not enough. Lorries should be in closed wagons and should be banned from carrying extra tanks of cheap fuel from the Continent.

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