Delays and cost increases to the Crossrail project will be investigated by the National Audit Office, it has been announced.
The central section of the project, which is building a new railway line underneath London, was due to open next month.
But in August it was announced that the opening of that section between Paddington and Abbey Wood had been delayed until autumn next year.
At the time, chief executive Simon Wright said more time was needed to “complete the testing of the new railway”, adding that employees were “working around the clock” on “one of the most complex and challenging infrastructure projects ever undertaken in the UK”.
In July this year, the government announced that the funding for the project had increased by £600m to £15.4bn.
In October, a further £350m of short-term finance was also made available for the project.
A spokesman for the National Audit Office said: “Our investigation will examine the causes of the cost increases and schedule delays, the terms of the additional funding, and the governance and oversight of the programme.
“We will also examine the steps being taken by Crossrail Ltd, TfL, and the department to minimise the impact of the cost increases and delays.”
The railway, which will be named the Elizabeth Line, is expected to serve around 200 million people each year, with many of its stations newly-built.
Once it is finally completed, Crossrail will take passengers from Abbey Wood, in London’s south east, to Heathrow Airport to the west of the capital.
Services are also planned to run from Shenfield in Essex to Reading in Berkshire.
The project is jointly sponsored by the Department for Transport and Transport for London with support from London’s business community.