Crossrail was driven over its budget and beyond its schedule after bosses clung to an unrealistic opening date, the public spending watchdog said.
Decisions were driven by a desire to meet the December 2018 deadline for the new east-west railway in London, the National Audit Office (NAO) said.
Crossrail cut risk management teams ahead of an expected completion, but is now rehiring them, the NAO said.
The company said it had taken the views of the watchdog “very seriously”.
The overall budget for Crossrail has risen from £14.8bn in 2010 to £17.6bn.
In response to delays in the schedule, testing of train and signalling systems were carried out in early 2018 even though “few meaningful results could be acquired at this point” because of a software problem, the report stated.
This took time away from those involved in construction work.
Changes to designs and contractors’ delivery schedules cost around £2.5bn between 2013 and 2018, according to the NAO’s analysis.
December 2018Original scheduled launch
March 2021Current expected completion, without Bond Street
15,000people have worked on Crossrail
60 milesDistance of the line from Reading to Heathrow
Amyas Morse, the head of the NAO, said: “Throughout delivery, and even as pressures mounted, Crossrail Ltd clung to the unrealistic view that it could complete the programme to the original timetable, which has had damaging consequences.
“While we cannot make an overall assessment of value for money until Crossrail is complete, there have been a number of choices made in the course of this project that have clearly damaged public value.”
Crossrail Ltd, the company building the railway, said services between Paddington and Abbey Wood will begin by March 2021, although Bond Street station will not be ready.
Full services across the line from Reading and Heathrow in the west to Abbey Wood and Shenfield in the east will begin “as soon as possible”, it added.
Crossrail Ltd chief executive Mark Wild said the company have taken the views of the NAO “very seriously”.
He added that a “new leadership team and enhanced governance structure has been put in place to strengthen the Crossrail programme and put the project back on track”.