Work on the tunnel is due to start on 17 January and last 12 months

The Strood Tunnel in Kent was shut for several hours on Friday after a driver noticed a pile of fallen chalk by the track.

The tunnel is due to close in January for building work due to last throughout 2004.

But union Aslef has called for talks with rail bosses over the safety of the tunnel.

Transferred onto buses

The driver who raised the alarm on Friday noticed the pile of chalk as he drove through the tunnel shortly before 0700 BST.

He completed his journey but after he raised the alarm the tunnel was closed until about 1500 BST when the debris had been cleared.

Engineers have spent the day investigating the problem while passengers had to be transferred onto buses.

A spokesman for Aslef, whose members include many rail workers, said the union wanted immediate talks with Network Rail and train operator Connex.

Succession of landslides

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, which runs through chalk land.

In May Network Rail announced the two-mile tunnel would close in January to be completely rebuilt into a steel and concrete structure, with rails and rail beds replaced and a new drainage system built..

Passengers on the line into London's Charing Cross will have to leave the train to catch a bus for part of their journey which Connex said would add 30 minutes to journey times in both directions.