Multi-million pound plans for modern trains along the South Coast could leave commuters on the daily grind between London and Brighton facing an extra squeeze.

While the coastal services are seeing an improvement as new rolling stock comes on line, travellers to the capital will get their shorter hand-me-downs.

Rail operator South Central is having to scrap the worst of its old slam-door trains running from Brighton to London.

The old fleet from the coastal lines is being used as a temporary replacement until its own new carriages, many of which are still on order, are introduced.

The problem is the trains are one carriage shorter - losing 70 seats per service.

In the past, trains have often been made up of two or three lots of four-car units, making trains eight or 12 carriages long.

But on some services one or two of these units are being replaced with three-car units, making trains seven, ten or 11 carriages long instead.

South Central spokesman Marsid Greenidge said: "We are taking some of the four-car, oldest slam-door trains which are normally used on services going to London and they are being replaced with slightly newer three-car units attached to four-car units.

"In the long-term, on all of our services we will have the longer trains but in between there is going to be a pinch point."

Mr Greenidge said it was just an interim measure because it was impossible to introduce all new rolling stock in one go.

He apologised for the inconvenience it could cause to travellers but said South Central was targeting services outside rush hour to minimise disruption.

He said: "We are trying to put them into the services that are not as busy.

"We will eventually have four-car trains again because we have ordered them for the new rolling stock.

"But there is going to be a period of time between getting rid of the old ones and introducing the new ones.

"We are reviewing it but we hope to have all the trains in place by the end of 2004."

Shelley Atlas, chairwoman of the Brighton Line Commuters, said as long as it did not impact on rush-hour trains it was a step forward.

She said: "The idea of getting rid of old rolling stock and replacing it is good.

"If the rail company is looking sensibly at services where it is not going to have such an impact, such as off-peak where not all the seats are filled, then that is okay.

"I would be very concerned if it was affecting commuter trains."