The duo will compete against Bovis Lend Lease for developer Chelsfield's project, to be called the Grand Union Building. Carillion, Sir Robert McAlpine and Skanska have dropped out of the two-stage tender.
A Mace spokesperson said the firm could not comment on the tie-up with Laing O'Rourke, although project insiders said the deal was imminent. The spokesperson denied claims that it had submitted a separate construction management bid.
Mace has been appointed project manager on the scheme.
One insider said the competition was neck-and-neck between Bovis Lend Lease and Laing O'Rourke/Mace.
He said: "They both have first-class teams. It would come down to the toss of coin to choose between them."
The Laing O'Rourke/Mace alliance echoes Mace's decision to link up with Sir Robert McAlpine last year. The two worked on the More London project, office schemes next to the Greater London Authority headquarters by Tower Bridge.
A revised planning application for the Paddington project was submitted by Chelsfield in November after criticism of the height of the proposed scheme.
The building was reduced to 12 storeys from 30 storeys after objections from the Royal Parks Agency and English Heritage that it was visually intrusive.
The scheme, part of the Paddington Basin regeneration project, now includes six interlinked blocks of nine, 12, 15, 18, 24 and 30 storeys.
The firm is tipped to beat off Structuretone, Mace and Interior to win the construction management contract at the bank's offices at 5 Canada Square.