The project budget was increased from £14.8bn to £15.4bn in July 2018
Crossrail, to be the Elizabeth line on opening, is a new railway for London and the southeast, running from Reading and Heathrow in the west, through 42km of new tunnels under London to Shenfield and Abbey Wood in the east.
The difficulties in the commissioning process centre on bringing together three different signalling systems with the new rolling stock. Testing has started, but further time is required to complete the full range of integrated tests to ensure the line opens as a safe and reliable railway, Crossrail said.
Crossrail chief executive Simon Wright said: “The Elizabeth line is one of the most complex and challenging infrastructure projects ever undertaken in the UK and is now in its final stages. We have made huge progress with the delivery of this incredible project but we need further time to complete the testing of the new railway. We are working around the clock with our supply chain and Transport for London to complete and commission the Elizabeth line.”