Crossrail has appointed US construction giant Bechtel as project delivery partner to the £15.9bn scheme
Having scooped the £400m prize, Bechtel will now be responsible for overseeing the central, twin-tunnelled section of the cross-London railway and will also manage the design process.
Bechtel joins Transcend Team on the project.
The joint venture, which includes fellow US firms, Aecom, CH2M Hill and Nichols Group, was confirmed as Crossrail programme partner last week.
The latest news follows speculation that the announcement was being delayed because of concerns over giving so much work to US contractors in the current downturn.
London mayor Boris Johnson played down any concerns of another US firm walking away with a big slice of work.
He said: “We anticipate that the vast majority of the jobs required to fulfil these contracts will be created in London and the UK, which can only be good news for the capital.”
At its height, more than 14,000 people are expected to work on Crossrail.
When completed in 2017, Crossrail will link Maidenhead in the west to Abbey Wood in the east.