Costain and Taylor Woodrow have been given the preliminary award for the contract to build a station at London Bridge, while Metro-Cammell, the train-building subsidiary of GEC, is believed to have won the contract for building new rolling stock for the 10-mile track.
Sir Robert McAlpine is part of a joint venture believed to have been chosen to build the link from Canary Wharf to Canning Town.
London Underground has sent out preliminary letters of intent for pounds 600m of the pounds 1.8m project, covering two of the 12 main construction sections, rolling stock, signalling and power supply.
These will become firm contracts when the Government gives the go-ahead for the extension, and work should start within weeks.
The project has been dogged by uncertainty since Olympia & York, the Canadian property company behind the Canary Wharf development, sought protection from its creditors in May. It had agreed to pay pounds 400m towards the cost of the project.
Canary Wharf's administrators said they could not afford the financing, but the Government insisted it could not go ahead without a private sector contribution.
The 11 banks owed more than pounds 500m on Canary Wharf have, however, agreed to take over O&Y's contribution. They will pay about pounds 100m immediately and the remainder over 25 to 30 years, equal to about pounds 170m in today's money.Reuse content