UK capital’s latest train project will be at least 3 years late, with the budget edging towards $20.5bn.

London’s new flagship rail line running from Heathrow Airport under the capital to the Canary Wharf financial district will cost more than expected and open later than planned in the latest setback to the $20bn project.

Billed as Europe’s most ambitious infrastructure project, the line was supposed to open in December 2018, but has been repeatedly delayed by problems with safety testing and the completion of signalling work.

Transport for London said on Friday it had now been notified that the line would not open until 2021. The budget is running at 15.4 billion pounds ($19.7bn) but could be between 400 million pounds ($512m) and 650 million pounds ($832.5m) higher than that.

The transport department said the route, named the Elizabeth Line, would open as soon as practically possible in 2021. It is in discussions with Crossrail about how to fund the additional costs.

“We are doing everything we can to complete the Elizabeth line as quickly as we can but there are no short-cuts to delivering this hugely complex railway,” said Mark Wild, Crossrail chief executive.

“The Elizabeth line must be completed to the highest safety and quality standards.”