Sources close to the project said Carillion had beaten off the other shortlisted candidate, Amec, to land the contract to manage and maintain BT's property portfolio.
Last week, Building revealed that the race to win the package was between Carillion and Amec after other shortlisted bids from Bovis Lend Lease/Dalkia and Mace/Citex were dropped by BT.
BT was understood to be keen on splitting the giant contract between Amec and Carillion. However, sources now say it has opted to place the whole package with one company.
A spokesperson for BT said: "Negotiations for Project Jaguar are still ongoing. No decision has been reached." Carillion was unavailable for comment. Amec declined to comment.
A source close to the project said BT had chosen Carillion because of the firm's experience in facilities management.
Originally, Project Jaguar was understood to include more than £200m worth of capital work, but this was later scaled down by BT.
The source said that, now the project was without the construction element, BT felt that Carillion had the type of experience in facilities management that was required.