Brennan was involved in the altercation while warming up during the match, and threw at least one punch to a fan in the Ulster section, leaving him requiring hospital treatment.

Brennan was swiftly yellow-carded after coming on as a substitute, and did not return to the pitch when his ten minutes were up, with Gregory Lamboley going on instead.

Ulster confirmed they had asked ERC, the Heineken Cup's governing body, to look both at the incident with the spectator and an on-field exchange with Ulster lock Justin Harrison which saw both players sent to the bin.

But the French club have now come out to defend their player, who they claim was the subject of unacceptable abuse while warming up as a replacement.

"The supporters were clearly over-excited and began chanting, 'Brennan, your mother is a whore'," the club said.

"Brennan approached this group and was showered with beer. The main gathering of Ulster supporters, who displayed exemplary sportsmanship, intervened between the group and Brennan, advising the player not to pay any attention.

"Brennan resumed his warm-up routine, but one member of the group began provoking him again. That was the point at which Brennan had his altercation.

"This is the first incident of this kind in our experience. Trevor is a generous player who has always respected his opponents and their supporters.

"It required sustained and repeated provocation to make him react in an unacceptable, but understandably human way.

"It is words and attitudes that cause the deepest wounds and we ask everyone to be aware of this, so that this remains an isolated incident in a sport that has always placed an emphasis on the values of friendship and camaraderie.

"Such supporters do not have a place in the sporting world, let alone in a rugby stadium. It falls to everyone to take their responsibilities seriously and we ask the organisers of the competition to instigate an inquiry into the behaviour of certain Ulster supporters," the club concluded.

The ERC have, meanwhile, confirmed that a disciplinary investigation is underway, as they look to get to the bottom of the complaints.