Brennan, who was contracted to French club Toulouse, has made the decision just days before he is due to face an independent disciplinary committee relating to allegations that he hit a supporter when Toulouse played Ulster.

Brennan is alleged to have entered a seating area during Toulouse's 28-13 European Cup victory over Ulster last month and punched a supporter.

There is a chance his professional career would have been brought to an end with a lengthy ban at the hearing, on Friday, but the lock has pre-empted any such decision.

Brennan had tried to have his ERC hearing adjourned pending a separate civil case but that appeal was turned down, forcing him to make the announcement.

He said: I had decided at the beginning of this season that I would retire at the end of the current French Championship.

However, as a result of recent matters I've had to review that position. I had sought an adjournment of the ERC disciplinary hearing on the grounds that it interfered with my right to silence in other proceedings, but this has been refused.

I am therefore announcing my immediate retirement from rugby. I would like to thank everybody who has helped me throughout these years, he added.