Cost of Great Western rail line modernisation spirals to £5.6bn
The spiralling cost of modernising the Great Western rail line has reached £5.6 billion, according to the public spending watchdog.
The figure represents a £2.1 billion increase since 2013, the National Audit Office (NAO) said.
It found that the project's delays now stretch to at least 18 to 36 months, costing the Department for Transport (DfT) up to £330 million.
The value for money of the work n eeds to be reassessed and the extent of electrification reconsidered, the report said.
NAO chief executive Amyas Morse said: " This is a case study in how not to manage a major programme.
"The department's failure to plan and manage all the projects which now make up the Great Western Route Modernisation industry programme in a sufficiently joined-up way, combined with weaknesses in Network Rail's management of the infrastructure programme, has led to additional costs for the taxpayer."
On Tuesday, rail minister Paul Maynard was accused by Labour of being responsible for " another broken Tory promise on rail" after the electrification of four sections of the Great Western line was "deferred".
The NAO accused the DfT of failing to plan and manage all the projects which now make up the Great Western modernisation programme in a coherent manner before 2015.
Last year Network Rail amended its plans for the scheme after it became clear that costs were increasing and the schedule could not be met.
The delay means that some passengers in the north and west of England may have to wait longer for improvements in their services, as the reallocation of Great Western trains is held up, the report found.
The report found that work to electrify the line between Maidenhead and Cardiff accounted for £1.7 billion out of the £2.1 billion increase in the cost of the programme.
In May, then-rail minister Claire Perry confirmed that all the new rolling stock ordered for the Great Western line for 2017-18 will be bi-mode, meaning it can run on diesel as well as electric power.
The 21 Class 801s were originally due to be entirely electric, but the change was made amid delays to the electrification project.
The NAO warned that compared with electric trains, bi-modes cause more damage to the track, incur higher energy costs as they weigh more and their top speeds are probably slower.
This calls into question whether the full extent of electrification is still value for money, the watchdog warned.
In March last year the DfT expected t he programme's benefit-cost ratio to be 2.4:1 but this should be lowered to 1.6:1, the NAO said.
A spokeswoman for Network Rail said: " The project continues to be challenging and complex but we are making good progress this year and there have been great successes in recent months including upgrading and electrifying the 130-year-old Severn Tunnel, and completing the Hinksey flood alleviation scheme, near Oxford."
A spokesman for Great Western Railway (GWR), which runs the franchise, said the operator "still expects to deliver its programme of customer improvements by 2019".
Ministers were accused of deceiving the public when the overhaul of the Midland Mainline and the TransPennine routes were "paused" weeks after the 2015 general election.
The work later resumed following a review by Network Rail chairman Sir Peter Hendy.
A Department for Transport spokesman said: "This report is clear about both the difficulties encountered by this ambitious project, and the improvements made by Network Rail and the department to manage this programme to make sure passengers get the improvements they want."