Amey had originally hoped to obtain up to £150m for the stakes but City sources believe they are only worth up to £50m.
The plan is for Laing to fund the equity stakes in the PFI contracts while Amey continues to provide the service.
This arrangement means the service contracts will not have to be renegotiated. The companies also plan to make joint bids for future PFI deals.
The deal does not include Amey’s stake in the Tube Lines consortium, which is bidding to refurbish one-third of the London Underground (see above). Amey has ringfenced that contract.