Work on the two-mile-long tunnel is due to last 12 months

The Strood Tunnel in Kent has been the scene of several chalk falls over recent years.

Network Rail plans to rebuild the two-mile-long tunnel, opened more than 150 years ago, as a steel and concrete structure.

From Monday, passengers will have to get off the train at Strood, travel by bus to Higham, and then get back on the train to complete their journey to London.

We are going to be able to get the speed back up to 70mph instead of this rather leisurely stroll at 20mph Michael Holden, South Eastern Trains

Rail operator South Eastern Trains said it would add an extra 30 minutes to journey times in each direction.

Robin Gisby, of Network Rail, said the year-long closure was necessary because of the difficult nature of the job.

He said: "It's a long way through there. There is an awful lot of work to do and access is not that easy.

"We really cannot go in and do it in the small hours, night after night."

Landslide problems

The Strood Tunnel was the scene of a major landslide in 1999 in which four carriages of a train were derailed.

A succession of smaller landslides since then led to a 20mph speed limit being introduced through the tunnel.

Michael Holden, of South Eastern Trains, said: "The work that's going to go on in here over the next year is going to give them another 150 years of life.

"We are going to be able to get the speed back up to 70mph instead of this rather leisurely stroll at 20mph."

The work by Network Rail will also mean the tunnel's rails and rail beds are replaced and a new drainage system built.

The tunnel was first proposed in 1778 and opened in 1824, when it was the largest in Britain.