Plans to almost double the number of Thameslink trains running through London have been delayed by a year, the Whitehall spending watchdog has said.
A timetable of up to 24 trains per hour at peak times was scheduled to start by the end of next year.
The National Audit Office (NAO) said it will be delayed until December 2019.
The Department for Transport and Network Rail had not made adequate plans to manage the timetables' launch, the NAO said.
Thameslink services, which run from Brighton to Bedford via central London, will be stepped up to 18 trains per hour in May with 20 per hour in December 2018.
Currently, up to 14 trains an hour run on the service during peak times.
The Department for Transport (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 £474m increase in the total budget for Network Rail's infrastructure works to £5.5bn as part of the £7bn Thameslink programme, the watchdog said.
Its 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: "Improving rail services is a priority for the government, which is why we are making the biggest investment in the network since Victorian times - including £7bn expanding the Thameslink network which will transform north-south travel across London and the south-east."