Forbes, a utility player who can operate virtually anywhere on the park, is believed to have agreed an 18-month deal with the club and will link up with Jackett for the second time, having previously played for him at Swansea City.
The 28-year-old goes straight into The Lions squad to face Walsall in the FA Cup and has been handed the number 20 shirt, with Peter Vincenti leaving to join Stevenage Borough.