Great Western rail electrification cost 'up to £2.8bn'
The cost of electrifying the Great Western railway line between Cardiff and London could reach £2.8bn, the boss of Network Rail has said.
Chief executive Mark Carne told MPs the estimate had been £874m in January 2013 and £1.5bn in September 2014.
Public accounts committee chair Meg Hiller called increase "staggering".
A senior civil servant also revealed there was now no firm completion date for the project, originally due to be finished by 2018.
Mr Carne blamed "inadequate planning and scope definition" in the early stages and an out-of-date cost base for the price increases.
"From the very early stages of the design of a project, as you go forward there's a significant degree of uncertainty," Mr Carne told the Commons public accounts committee on Wednesday.
"I fully accept that this is extremely disappointing to everybody involved," he added.
Ms Hillier responded: "It's unbelievable and it's unacceptable that there was such poor planning."
Department for Transport Permanent Secretary Philip Rutnam told MPs he was "not in a position to give you a schedule for Great Western electrification".
"You're probably aware of the schedule that there was - which was 2016 to Bristol Parkway, Newbury, Oxford, 2017 to Cardiff, 2018 to Swansea," he said.
"It is clearly highly likely that there will be delays against that schedule and I'm afraid I'm not in a position at the moment to give you a new schedule."