The firm will receive £66.3m in cash for its 10% stake from Edge Orbital Holdings 2, a consortium of institutional investors arranged and advised by Macquarie Capital (Europe).
Completion of the sale, which remains subject to various third party consents, is expected before the end of the current financial year.
In October, Skanska sold its 40% stake in the road widening PFI for £265m to a consortium of Equitix, Dalmore Capital and US fund manager GCM Grosvenor.
The M25 road-widening PFI is currently owned by Connect Plus (M25). The SPV also comprises Balfour Beatty (40%), which is also said to be looking to sell its stake, and EGIS Projects (10%).