The full introduction of new rail services to ease north-south travel through central London has been delayed by up to a year, the Whitehall spending watchdog has said.

A metro-style timetable of up to 24 trains per hour at peak times was scheduled to be in place by the end of next year, but the National Audit Office (NAO) revealed this has been pushed back.

Thameslink services will step up to 18 trains per hour in May and 20 per hour in December 2018, with the full increase to 24 per hour not reached until December 2019.

The decision was taken after the Department for Transport (DfT) and Network Rail failed to make adequate arrangements to manage the launch of the new services, the NAO said.

The DfT approved a proposal it had requested from Govia Thameslink Railway - which operates trains on the affected routes - to increase services more gradually in order to "manage the risks of each service change", the report stated.

There has been a £474 million increase in the total budget for Network Rail's infrastructure works to £5.5 billion as part of the £7 billion Thameslink programme, according to the watchdog.

It is not the first time that an increase in services on the route has been delayed.

A modernisation scheme named Thameslink 2000 was first discussed in 1991 but did not begin until 2006 due to privatisation of the railways and complex planning inquiries.

NAO head Amyas Morse said: "The Thameslink programme has a realistic prospect of delivering value for money, with significant benefits for passengers in terms of increased capacity and more trains, but a number of key risks and challenges still remain to be overcome.

"The Department's recent decision to introduce new services more gradually than originally planned means that passengers will get the full expected benefits one year later than planned, but it is a sensible step to protect value for money and passengers from further possible disruption due to large numbers of new services being launched at the same time."

A DfT spokeswoman said: "We welcome the NAO's findings that the Thameslink programme will deliver significant benefits for passengers and supports the recent decision to gradually introduce new services to protect passengers from disruption."

Chris Gibb, chairman of the Thameslink Programme Industry Readiness Board, said: "By phasing the introduction of the new timetable in this way, we have front-loaded the benefits for passengers and then spread further changes in such a way that they can be more reliably introduced."

A Network Rail spokesman said: "The Thameslink Programme is transforming north-south travel through London to provide more frequent, more reliable journeys to new destinations for passengers and upgraded stations including the landmark, entirely redeveloped London Bridge."