The platform is is being lengthened to accommodate longer trains

Officials said the work would allow operators to use longer trains to meet the growing demands of passengers.

From Monday until 1 September trains to and from Cardiff will start and terminate at Taffs Well and a replacement bus service will operate.

A reduced train service will connect Cardiff to the Valleys in September.

The buses will connect Arriva Trains Wales services which go through Pontypridd station and come into and out of Cardiff central station.

The services will operate to and from Taffs Well, Pontypridd, Aberdare, Merthyr Tydfil and Treherbert, calling at all the usual station stops in between.

Between 2 and 9 September, two trains an hour will operate between Cardiff and Treherbert and between Cardiff and Aberdare. Both services will call at all intermediate stations including Pontypridd.

It was decided to do the work now because at this time of year less people are travelling to work Arriva Trains Wales spokeswoman

An hourly train service will run from Merthyr Tydfil to Abercynon for passengers to change and catch the Aberdare service to Cardiff.

Details of the amended train services would be published on posters, said an Arriva Trains Wales spokeswoman.

The firm's commercial director Mike Bagshaw apologised for any inconvenience to passengers but said the work would improve the rail service.

He said: "From December this year Arriva Trains Wales takes delivery of an additional 18 carriages and the longer platforms will allow these to be added to our busiest commuter trains."

An Arriva Trains Wales spokeswoman added that the work had been scheduled to avoid the Rugby World Cup warm up game between Wales and France at the Millennium Stadium on Sunday.

"It was decided to do the work now because at this time of year less people are travelling to work," she said.

A replacement bus service is running while the work is carried out

"Our services will benefit from this work because we will be able to cope better with the numbers of commuters. It will reduce overcrowding."

Simon Pickering from Passenger Focus said that while it was unfortunate the work would cause disruption, it would benefit passengers in the long term.

"What is important is that passengers are informed of the changes and the work is carried out efficiently and does not overrun," he said.

The Welsh Assembly Government and the South East Wales Transport Alliance (SEWTA) are investing in the project, which is part-funded by the EU's Objective 1 programme.

Ieuan Wyn Jones, Deputy First Minister and minister for transport said he recognised that the work would result in a number of "short term disruptions to the service" but that they were working to ensure passengers experienced the minimum amount of disruption.