LONDON (Reuters) - London’s Crossrail project running from Heathrow Airport to the Canary Wharf financial district could cost an extra 650 million pounds and not open until 2021, more than two years behind schedule, Transport for London said.

FILE PHOTO: Former Britain's Prime Minister David Cameron (2nd R) and former London Mayor Boris Johnson (2nd L) visit a Crossrail construction site beneath Tottenham Court Road in central London January 16, 2014. REUTERS/Ben Stansall/pool

The $23 billion (£18 billion) Elizabeth Line, billed as Europe’s most ambitious infrastructure project, 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 be delayed further, and that its cost could rise to 18.3 billion pounds.

“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,” Crossrail CEO Mark Wild said.

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

When open, the line will connect destinations such as Heathrow in the west of the city to the rail hub of Paddington, central shopping districts Bond Street and Tottenham Court Road, Canary Wharf in the east, and beyond.

It is expected to carry about 200 million passengers a year and alleviate pressure on London’s 19th century underground network, known as the Tube.

The idea of a “Crossrail” line running across the city has been envisaged since the early 1970s but like other major infrastructure projects such as the expansion of Heathrow, it took years to get the project approved.

The plans slowly gathered steam in the early 2000s and parliament approved the project in 2008.

BBC TV has documented the progress of Crossrail, showing the work of thousands of engineers carving the route through the warren of existing tunnels under London. The line will stretch for more than 100 km and required 42 km of new tunnels.

The trains have been delivered and most of the new stations are close to completion, but work is ongoing on signalling software and train systems.

Equipment still needs to be installed in tunnels and then trains must complete test runs on thousands of miles of track to iron out any problems.

The UK’s Department for Transport said it is in discussions with Crossrail about how to fund the additional costs and the route would open as soon as practically possible.