Under the deal, Laing will retain a 22.5% stake in the arm but would also make £30m from the sale. Laing denied last week that it was intending to sell its UK housebuilding arm for more than £300m.
Speaking at the group's annual meeting, Sir Martin Laing said talks over the sale of the company's construction arm to subcontractor O'Rourke were continuing.
He added that due diligence problems over Morrison's acquisition by AWG (formerly Anglian Water) had slowed down the process, revealed in Building earlier this month.
The deal is due to be completed by the end of June.
He said: "Negotiations … have not been helped by the recent difficulties announced by Anglian Water arising from its purchase of Morrison."
Sir Martin said that Laing's investment arm was about to submit bids for the Thames and Wessex rail franchises later this year.