Some slam-door trains were first introduced in the 1960s

The companies - South West Trains, South Central and South Eastern - asked the Health and Safety Executive (HSE) for permission until November 2005.

Slam-door trains are due to be taken out of service by December 2004.

The HSE said it was "minded" to allow the companies to use the old stock as long as they were withdrawn "as soon as was practicable".

In 1999, the companies agreed to withdraw the old Mark 1 stock because of concerns over the doors and lack of protection during a crash.

The HSE wants to agree a programme, by the end of 2004, to put in place measures to minimise safety risks with older trains as part of the deal.

The HSE will now consult the rail industry before deciding whether to approve the plan.