A revised rail timetable promising improved services and more trains across the south of England has been marred by a wave of delays and cancellations that have also affected services between London and south Wales.

The railway companies GWR and Transport for Wales cancelled or curtailed a slew of trains on Sunday as the new timetable came into effect.

GWR had promised that its biggest timetable change since 1976 would bring “faster, more frequent services with thousands more seats across the region”.

However, it said it had been forced to amend services on the lines running from London to Swansea and Penzance on Sunday owing to “the unusually high number of rail staff are not available to work”.

Customers travelling from London to Plymouth or Penzance were advised to change to GWR’s local services at Exeter, where London trains were terminating.

GWR said services from Exeter and Plymouth “will be supported by additional road vehicles. adding up to an hour on journey times.”

A reduced service was operating between Paddington and south Wales. Trains from Paddington and Cheltenham were terminating at Swindon where travellers could catch another train.

Transport for Wales reported at least nine cancellations, which it attributed to a lack of staff and train faults. Among these was a new service connecting Pwllheli and Machynlleth.

A TfW spokesman said: “We are continuing to do everything possible to ensure the maximum number of services are operating every day, and this time of year can always be challenging, particularly on weekends, and we’re sorry that there have been a number of cancellations on our network.”

He said the company was recruiting additional staff and was confident that customers would see a improvement across its network.

Train timetables are changed twice a year, in May and December. According to the industry body the Rail Delivery Group (RDG), the most recent changes introduce the first non-stop trains between London and Bristol in decades and 1,000 extra services a week, on top of 4,000 added over the past two years.

A botched change in May last year led to chaos, and the passenger watchdog Transport Focus said travellers would be hoping for a smoother introduction of the latest changes.

The RDG sought to reassure passengers over the upcoming timetable, stating that the industry had put “years of work into drafting, consulting and planning for these changes”.

Robert Nisbet, the organisation’s director of nations and regions, urged passengers to check their journey details in advance as he said many times were changing.

He told PA Media: “Train operators and Network Rail will be working together to run a reliable service and respond quickly to any teething problems as people get used to the change.”

Improvements have been promised on the ScotRail network, with additional services in north-east Scotland and extra seats between Edinburgh and Glasgow. A new station, Robroyston, is due to open in north-east Glasgow on the line between Queen Street and Cumbernauld.

Other operators introducing new services are Greater Anglia, London North Eastern Railway, Northern, TfL Rail, Thameslink, Transpennine Express (TPE), West Midlands Railway and London Northwestern Railway.