The opening of London's Crossrail project will be delayed until 2021 as Europe's biggest infrastructure scheme is set to go another £650m over budget.
The route, to be known as the Elizabeth Line, was originally due to open in December 2018.
Crossrail Ltd chief executive Mark Wild said services would be delayed to allow time for more testing.
He also said the cost of the project could reach £18.25bn, more than £2bn more than the original budget.
The Department for Transport said it was "considering" where this new funding would come from.
The cost was originally set at £15.9bn for the scheme, which will connect major landmarks such as Heathrow Airport and the Canary Wharf business district.
However, Mayor Sadiq Khan, the government and Transport for London (TfL) had since agreed a figure of £17.6bn.
Bosses said in April that services would begin between October 2020 and March 2021.
Announcing the latest delay, Mr Wild insisted services would begin "as soon as practically possible in 2021".
He added: "The central section will be substantially complete by the end of the first quarter in 2020, except for Bond Street and Whitechapel stations where work will continue.
"We will provide Londoners with further certainty about when the Elizabeth line will open early in 2020."
The delay will allow more time to complete software development and allow safety systems to be tested.
By Tom Edwards, BBC London transport correspondent
Just weeks ago I spoke to businesses up and down the Crossrail line and there was very little confidence with anything they were being told by the company.
Well they were right. We are getting a drip-drip of delay and uncertainty.
Crossrail's hopeful "opening window" of between October 2020 and March 2021 just got slammed shut. Now it'll open "as soon as possible in 2021".
And then there's the extra cost. I suspect that will again come from London businesses through the precept, which was meant to be earmarked for other transport projects like the second phase of Crossrail.
Yes, this will be an incredible project when it's finished. But at the moment businesses will despair.
The line will make use of some existing track, but involves 26 miles of new tunnels connecting Paddington and Liverpool Street stations to improve rail capacity crossing the capital.
Eventually it will connect London to Reading and Shenfield.
Mike Cherry, chair of the Federation of Small Businesses, said the delay to the service is "fast-becoming a national embarrassment".
He said: "Another delay to Crossrail is disappointing to see and will leave a sour taste in the mouths of businesses and commuters who had been preparing for the start of the service in 2020."
Mr Khan said he was "deeply frustrated" by the new delay.
A spokesman for the Mayor said "Further work is taking place immediately to assess Crossrail's latest cost estimates.
"TfL and the Department for Transport, as joint sponsors, will continue to hold the Crossrail leadership to account to ensure it is doing everything it can to open Crossrail safely and as soon as possible."
An estimated 200 million passengers will use the new underground line annually, increasing central London rail capacity by 10% - the largest increase since World War Two.
Crossrail says the new line will connect Paddington to Canary Wharf in 17 minutes.
In May, Crossrail was criticised by the National Audit Office for running late and over budget, suggesting that bosses had clung to an unrealistic opening date.
A TfL spokesman called the delay "disappointing".
In a statement it said: "It is only over the last year that the new Crossrail leadership has established the full complexity of finishing the software development and signalling systems, while getting the necessary safety approvals to complete the railway.
"Full testing is due to get under way next year and there can be no shortcuts on this hugely complex project."