The capital’s new Crossrail service, known as the Elizabeth line, will open on Tuesday 24 May, Transport for London said.
The railway will run from Reading in Berkshire to Shenfield in Essex and Abbey Wood in southeast London.
It was was originally planned to open in full in December 2018 but it was hit by numerous problems including construction delays and difficulties installing complex signalling systems.
It is hoped the Queen – who the line is named after – will be involved in the opening.
The Elizabeth line will operate 12 trains per hour between Paddington and Abbey Wood from Monday to Saturday 6.30am to 11pm.
Further testing and software updates will take place on Sundays.
The Sunday closures will be lifted on 5 June to help people travelling in the capital during the Platinum Jubilee weekend.
Andy Byford, TfL’s commissioner, said: “I am delighted that we can now announce a date for the opening of the Elizabeth line in May.
“We are using these final few weeks to continue to build up reliability on the railway and get the Elizabeth line ready to welcome customers.
“The opening day is set to be a truly historic moment for the capital and the UK, and we look forward to showcasing a simply stunning addition to our network.”
The project went massively over its original £14.8bn budget in 2010.
The total cost of the project has been estimated at £18.9bn, including £5.1bn from the government.
Mayor of London Sadiq Khan said the Elizabeth line will make London “safer, fairer, greener and more prosperous”.
He went on: “This is the most significant addition to our transport network in decades, and will revolutionise travel across the capital and the South East – as well as delivering a £42bn boost to the whole UK economy and hundreds of thousands of new homes and jobs.
“Green public transport is the future, and the opening of the Elizabeth line is a landmark moment for our capital and our whole country, particularly in this special Platinum Jubilee year.”
A full timetable with direct trains running across the Elizabeth line is expected to launch by May 2023.
Bond Street Elizabeth line station will not be ready to open on 24 May but is expected to be completed by the end of the year.