The Wales forward received his marching orders after punching Castres replacement Mauricio Reggiardo in the 83rd minute.

Thomas became frustrated after being held by two Castres defenders, hitting out in front of a linesman and promptly being dismissed by referee Dave Pearson.

With back-row forward Andy Lloyd being taken to hospital in a neck brace, albeit as a precautionary measure, the Ospreys suffered a massively difficult evening southern France.

They went into the game unbeaten and top of the Celtic League after seven straight wins, but were overrun by four Castres tries.

Former Wasps Mark Denney and Paul Volley stung the Welsh in the first half, then Benjamin Lhande went over and the Ospreys conceded a penalty try.

Adam Jones and Jason Spice responded with scores, but Lyn Jones’ side were always struggling.

The Ospreys had been forced into a late reshuffle after their prime ball carrier this season, No.8 Ryan Jones, pulled out with a rib injury.

Lyndon Bateman came in at lock, Thomas switched to blindside flanker and Andy Lloyd took over from Jones in the middle of the back row.

Bateman started well, stealing two lineouts on Castres’ throw, but the French outfit were not long in imposing themselves.

In perfect conditions, they built a 20-0 lead in 27 minutes to leave the Ospreys looking down and out.

The French side exerted an iron grip up front, where they repeatedly forced the Ospreys on to the back foot with their impressive driving maul.

They also had a master controller behind in fly-half Yann Delaigue, who had time and space to choose his options as the visitors struggled to get near him.

Delaigue fired his side in front on seven minutes after the Ospreys had been penalised for shoving at a lineout.

Midway through the half, the former Parker Pen Shield winners extended their advantage after a brilliant move that started with a lineout take by Nicolas Spanghero.

Flanker Romain Froment powered clear into the heart of the Ospreys’ defence before, Mario Ledesma drove on to the line before Delaigue sent Denney over.

Minutes later Volley made it over after another immense surge from Froment that scattered two defenders. As the Ospreys struggled to track back, Volley was on hand to claim the score.

With Delaigue converting both tries and adding a penalty the visitors were looking down both barrels.

But they fashioned a response thanks largely to a marvellous piece of play from Sonny Parker.

The Wales centre opened the French defence with a piercing break which was eventually stopped 15 metres from the line. But Lloyd supported well to put Adam Jones over from short range.

The Ospreys were still adrift 23-7 at the break, though, a deficit that grew wider as Lhande crossed six minutes into the second period.

The Welsh side’s problems only grew as Bateman was yellow-carded for tackling Ugo Mola without the ball as the full-back raced clear.

And while they were down to 14 men, the Ospreys conceded again when referee Dave Pearson awarded a penalty try after the visitors collapsed a close-range scrum.

The Welshmen hit back through Jason Spice, who profited from a break from Gavin Henson to claim a try that crowned an impressive personal show.

But then came Thomas’ moment of madness.