The House of Commons Public Accounts Committee (PAC) has published the findings of its inquiry into Network Rail's 2014-2019 investment programme.
It has highlighted the "staggering and unacceptable" cost increases throughout the programme, which aims to electrify the Great Western Main Line from London to Cardiff.
However, the project is now expected to cost up to £1.2bn more than the £1.6bn estimate made in 2014.
In addition, the committee stated there is "far too much uncertainty" on the costs and delivery dates for both the TransPennine route and the Midland Main Line electrification projects.
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It warned other projects could now be delayed in order to balance Network Rail's budget.
Meg Hillar, Chair of the PAC, said the organisation had "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," she said.
"The potential near-doubling in cost of the electrification of the Great Western Line is a symptom of seriously flawed control and planning. Another is the continuing uncertainty over electrification of both the TransPennine route and the Midland Main Line.
"The government has identified rail infrastructure as a vital part of its economic plans, for example in establishing what it describes as a ‘Northern Powerhouse’. It is alarming that, in planning work intended to support these plans, its judgement should be so flawed.
"Our inquiry has found that the agreed work could never have been delivered within the agreed budget and timeframe. Yet Network Rail, the Department for Transport and the regulator – the Office of Rail and Road – signed up to the plans anyway."
(LM/MH)
