Thousands of West commuters could be left with standing room only on local trains because the Welsh want their trains back, according to campaigners.

Passenger watchdogs fear Welsh Assembly Transport Minister Ieuan Wyn Jones is poised to ask First Great Western (FGW) to return 10 carriages they have on loan.

They say returning the carriages will have a devastating impact on commuters travelling around Bristol and between Wales and the West Country.

"If we lose 10 carriages we will not be able to sustain the Bristol suburban rail network and services into South Wales," said David Redgewell of Transport 2000.

"If they cannot get any more rolling stock to replace it then there will be severe overcrowding and a repeat of past problems. People could be left standing on platforms. Mr Wyn Jones would do well to remember that Welsh passengers use these services and unless the national boundaries have changed, Monmouthshire is still in Wales."

Arriva Trains Wales agreed to lend FGW the carriages on a short-term basis as part of a £29-million rescue package demanded by former transport minister Ruth Kelly to help the 79 million beleaguered passengers who use the West Country services every year.

The deal was designed to increase carriages on the Cardiff to Bristol to Portsmouth service, but rail bosses have done some juggling and used them to boost services on the Cardiff to Taunton and Malvern to Weymouth line and the trains stop at stations across Monmouthshire, Somerset, Wiltshire and Gloucestershire.

The decision to lease the carriages caused a row in the Welsh heartlands and only this week the boss of Arriva Trains Wales, Tim Bell, told a committee of MPs that Welsh services were overcrowded and needed more trains.

Mr Wyn Jones recently announced 2000 more seats on train services between Cardiff and the South Wales valleys and publicly pledged to add extra trains to the Merthyr to Cardiff Service from May 2009. Campaigners say he will not be able to deliver on the May promise without asking FGW to return the Welsh trains.

Councillor Barbara Lewis, of the West of England Partnership, is set to write to the Welsh Assembly, which subsidises Arriva Trains, to find out about their future plans.

A partnership spokeswoman said: "If the rumours are true and if that was to happen, then we would be very disappointed. We need to clarify the situation and a letter will be going out shortly."

Last month it was reported that FGW was seeking to renegotiate its £1.1-billion contract with the Government so it could obtain more rolling stock, which can cost up to £2m per carriage.

FGW spokesman Adrian Booth said there was closing date on the leasing deal with Arriva but the Welsh train company had not yet asked for their trains back.

"We have not heard anything, but if we do we will have to consider our options," he said. "There really isn't enough rolling stock to go around and whenever it becomes available there is fierce competition between the various operators.

"Like all train operators throughout Britain, we simply do not have enough stock to cope with rising passenger numbers."

A spokesman for the Welsh parliament said: "The Welsh Assembly government is currently discussing the rolling stock requirements for the additional services to Merthyr from May 2009 with Arriva Trains Wales."