Crossrail, the new east-west London railway designed to bring relief to jam-packed commuters, announced Friday that it is postponing its scheduled December opening by the better part of a year.

Crossrail Ltd. said services on the central section of the line, between Paddington station in west London and Abbey Wood in the east, will begin "in autumn 2019," with the rest of the line opening as soon as possible after that.

It said in a statement that the railway — which will be named the Elizabeth Line after Queen Elizabeth II — needs more time to finish "final infrastructure and extensive testing."

The 73-mile (118-kilometer) line is one of Britain's biggest infrastructure projects for decades and has cost around £15 billion ($20 billion). It includes a 13-mile (21-kilometer) underground section through the heart of the city that has taken years to build.

Crossrail chief executive Simon Wright said the new line "is one of the most complex and challenging infrastructure projects ever undertaken in the U.K."

Crossrail Elizabeth Line
Britain's Queen Elizabeth and Mike Brown the London transport commissioner attend the formal unveiling of the new logo for Crossrail, which is to be named the Elizabeth line, at the construction site of the Bond Street station in central London, February 23, 2016. R EUTERS/Richard Pohle/Pool

The firm said work on the central tunnels and on developing software had overrun, cutting into the scheduled testing time.

Builders say Crossrail will add 10 percent to central London's rail capacity — easing the journeys of commuters who now pack the Underground's overstuffed Central Line — and bring a £42 billion ($55 billion) boost to the economy.

The decade-long construction project has also been one of London's biggest archaeological digs, uncovering everything from 68,000-year-old mammoth bones to the remains of a Tudor manor house and the skeletons of 14th-century Black Death victims.