The government has been blamed for rising costs and delays to the “modernising” of the Great Western railway, which could threaten the electrification of some parts of the route.

The National Audit Office said the cost of modernisation is estimated to be £5.58 billion, an increase of £2.1 billion since 2013 and delays of at least 18 to 36 months.

The government has said part of the project would be “deferred” with money saved on the project going on “additional benefits for passengers”.

The plan was to fully electrify the route from Cardiff to London by 2018 and the scheme had already been delayed.

Lines affected include major routes to Bristol, Oxford, and Berkshire.

“Network Rail’s 2014 cost estimate was unrealistic,” said the NAO in its report. “It was too optimistic about the productivity of new technology. Failings in Network Rail’s approach to planning and delivering the infrastructure programme further increased costs.

“It did not work out a ‘critical path’ – the minimum feasible schedule for the work, including dependencies between key stages – before starting to deliver electrification. It also did not conduct sufficiently detailed surveys of the locations for the structures, which meant that some design work had to be repeated.”

Andy McDonald, Labour’s shadow transport secretary, accused the government of “incompetence.”

He said: “These electrification works have seen their cost triple and have now been shelved twice, with businesses and commuters being made to pay the price for the Government’s incompetence.

“The Government have proven themselves incapable of delivering electrification works to budget or on schedule, and this is yet another example of a broken Tory promise on rail.”

Sign up to BBT’s twice-weekly newsletter

Subscribe to the BBT Newsletter

Join the Buying Business Travel newsletter for the latest business travel news.

Thank you for signing up!