Euro 2012 qual. - Bent doubt for Bulgaria clash
Bent had missed the previous two days' sessions with a groin problem and he again failed to participate in the final session 24 hours before the game.
The Aston Villa player also missed last month's friendly international against Holland - subsequently postponed because of the London riots - with a shoulder setback.
The former Sunderland player had looked to be establishing himself as a regular member of Fabio Capello's attack after a spell of three goals in three matches.
But his possible absence on Friday could open the door for Jermain Defoe, who scored a hat-trick against Bulgaria at Wembley a year ago.
Bent was the only absentee from training as England finalised their preparations for two games in four days, which will go a long way to determining whether they gain automatic qualification to next summer's finals.
They will follow up the trip to Sofia with a home encounter against Wales at Wembley on Tuesday.