Commuters face 18 months of disruption across Sussex and into London as engineers boost the power supply so new trains can run properly.

Hundreds of temporary line closures will be needed during the upgrading of electrical supplies to enable new Electrostar trains to travel at full speed.

Throughout the South of England, Network Rail says it needs 1,500 such closures to allow work on replacement of cables, conductor rails and connecting up of new sub stations.

Extensive work is under way at Brighton, Hove, Hassocks and Burgess Hill, and also Clayton Tunnel between Preston Park and Hassocks.

Three new sub stations, which take 33,000 volts from the National Grid and reduce them to the 750 volts which are put into a third rail to power the new trains, are being built at Hove, Brighton and Littlehampton.

Line closures will also be required before these sub stations are connected up.

Network Rail, which took over the responsibility of maintaining Britain's rail network from Railtrack, has promised to keep disruption to a minimum, with most of the work being done in the early hours of the morning and weekends.

It will mean the cancellation of some early-morning and late-night services with buses replacing trains on certain routes at weekends.

There will also be shutdowns for platform extensions and signalling improvements. It is hoped everything will be completed by mid-2005.

A spokeswoman for Network Rail said: "We will need track possessions in the night and at weekends.

"It is not possible for us to complete some of the work during the night as by the time you have closed the line and got all equipment on site, not many hours are available for our teams to work.

"As a result, we need more time at weekends. We are working with the rail companies to keep disruption to a minimum.

"We will also try to give as much warning to the travelling public as possible.

"We would advise passengers travelling at weekends to check about line closures with National Rail Inquiries on 08457 484950."

Rail companies do have an input into the timings of closures and have already made objections to those which were scheduled to take place while sporting and other big events took place.

This weekend no track closures are scheduled for the Brighton Line or lines in East Sussex.

But the line between Barnham and Angmering will be closed. Bus services are available for people travelling along the coast between Brighton and Portsmouth.