A committee of MPs have criticised the rising costs of the electrification of the Great Western Rail line, describing them as "staggering and unacceptable".
In 2014, the Office of Rail and Road (ORR) initially said the work between Maidenhead and Cardiff would cost £1.6 billion. But that figure is now estimated to be £2.8bn. The Public Accounts Committee said Network Rail had "lost its grip" on large scale projects, but that it was not clear how or why.
The Rail organisation accepted that planning had not been good enough, but stressed that changes have been implemented to reduce soaring costs.
Rail electrification is designed to improve journey times. It is hoped that shorter train journeys will encourage more domestic tourism and make it easier for businesses to travel to and from London, boosting the UK economy in the long-term.
Network Rail had set the London to Oxford and Bristol Parkway parts of the line were due to be electrified by 2016, extending to Cardiff in 2017 and Swansea in 2018. But now, the Public Accounts Committee says there is no estimated dates of completion.
Meg Hillier, who chairs the committee, said: "Network Rail has lost its grip on managing large infrastructure projects.
"The result is a twofold blow to taxpayers: delays in the delivery of promised improvements and a vastly bigger bill for delivering them."
The MPs also criticised the ORR for not being tough enough in its scrutiny of Network Rail.
An ORR spokesperson said: "We need to learn the lessons, and agree with PAC's recommendations that uncertainties in key projects need to be addressed differently."
A National Rail spokesperson said: "Network Rail has successfully delivered over 5,000 projects over the past five years, but our understanding of how best to plan and deliver major new electrification schemes was not good enough.
"We have now made significant changes to the way we plan and deliver our investment programme, which will see schemes progress only once they are sufficiently developed that a reliable cost estimate can be established."