Saracens must play their Investec Champions Cup round-of-16 match away from StoneX Stadium after they were stunned 32-24 by Castres.
With both teams having already qualified for knockout phase, the clash in north London was a shootout for home advantage – and the three-time champions fell to a major upset.
Castres named an entirely new starting XV following their 49-10 demolition of the Bulls last weekend and, in defiance of their reputation as poor travellers, they sealed the win that clinched second place in Pool Three.
A 24-17 lead established early in the second half hinted that Saracens had weathered the storm from the Top 14’s eighth-placed team, with England forwards Theo Dan and Ben Earl crossing twice each.
But Castres kept on coming and showed greater appetite for the fight when the match hung in the balance, although the warning signs that they meant business were apparent right from the start.
Match Summary 
Only two minutes were on the clock when Andrea Cocagi bulldozed through the home defence with number eight Feibyan Tukino arriving to complete the score, but the try provided the jolt Saracens needed.
After probing dangerously, they crossed when Dan took a quick penalty and stormed over.
Saracens were slowly getting on top with their scrum particularly effective, but when they were turned over close to the whitewash Castres issued a reminder of their threat through a dynamic run by Josaia Raisuqe.
Vulnerabilities in the home defence were being cleverly exploited as Castres moved downfield and in the 27th minute they struck for the second time, with Adrien Seguret charging over from a free-kick move.
Saracens were foiled inches short of the line by a turnover and then Earl spilt forward with players lining up to continue another promising attack.
Pressure was building on Castres with their scrum infringing once again and they were breached by a route-one approach, with Earl the last of several forward drives.
Saracens were then presented with a very kickable penalty but boldly opted for touch and were rewarded with a line-out drive that ended with Earl touching down for his second try.
It gave them the lead for the first time and a 19-17 interval advantage, which was quickly extended when play resumed with Dan finishing another line-drive.
But they could not afford to relax as Castres hit back, Cocagi offloading out of the tackle to Raisuqe and once the Fijian wing had bounced off Elliot Daly, Theo Chabouni was on hand to complete a thrilling riposte.
And Saracens were undone again, although this time their defence was glaringly at fault as replacement scrum-half Santiago Arata Perrone danced into space, weaving around tacklers, before Seguret cut back inside to score.
Play was held up after substitute back row Toby Knight received treatment for a significant injury and, when the action resumed, Castres withstood an attack and then produced a key 50-22.
With seconds remaining, Louis Le Brun kicked a penalty to send the visitors eight points clear, completing a deserved win.
