The 22-year-old attacking midfielder, whose Brentford contract has expired, rejected a new deal at Griffin Park and was keen to secure a move to the Championship club.
Rovers will announce Forrester’s signing later today.
As the player is under 24, he cannot move on a Bosman free transfer so a tribunal will set a compensation fee unless the two clubs agree a deal.
Forrester, signed in 2011 after being released by Aston Villa, made 55 league appearances for the Bees, scoring eight goals.
Source: Donny Mad