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Investec Champions Cup Pool 3
Saracens 24 (19)
Tries: Dan 2, Earl 2 Cons: Lozowski 2
Castres 32 (17)
Tries: Tukino, Seguret 2, Chabouni Cons: Le Brun 3 Pens: Le Brun 2
Three-time winners Saracens fell to a shock 32-24 defeat by Castres as they missed out on a home tie in the last 16 of the Investec Champions Cup.
Castres had not won away in Europe for 12 years heading into the final match in Pool 3 and changed their entire starting XV after last week's win over the Bulls.
In Maro Itoje's first game since being appointed England captain, Saracens led through two tries apiece from forwards Theo Dan and Ben Earl.
But Castres took the lead in the closing stages thanks to a second try from captain Adrien Seguret.
Saracens, who struggled to handle Castres' physical backs, were unable to score the converted try that would have snatched victory.
Castres, who appointed Xavier Sadourny as head coach earlier in January, sealed a deserved win with a late penalty.
Northampton Saints won the pool with Castres moving ahead of Munster to finish in second place, the first time the French side have reached the knockout stage since 2002.
Saracens go through to the last 16 as the fourth-place qualifiers.
Saracens director of rugby Mark McCall had warned before the match that this was "probably the biggest game" in Castres' European history, and he was proved right as the visitors rose to the occasion.
The first of several powerful runs from Fijian Andrea Cocagi, who crashed through some suspect Saracens defence, gave Feibyan Tukino the chance to score the opening try in the third minute.
Hooker Dan, starting ahead of deposed England skipper Jamie George, took a quick-tap penalty and powered his way over the line to bring the hosts level.
Castres regained the lead from a well-executed penalty play which ended with centre Seguret smashing his way over for his first.
Saracens' pack clearly had the upper hand in the scrum and line-out and used their power to help Earl over for the first of his tries.
The home side's forwards repeated the trick as Earl grabbed his second on the stroke of half-time, as Sarries took the lead for the first time.
Dan, who carried Saracens forward with his bullish runs, got his second try from another maul as the home forwards pushed Castres back again.
But the visitors finally steadied their set-piece and that gave them the platform to unleash their dashing backs.
Cocagi's brilliant back-of-the-hand pass released fellow Fijian Josaia Raisuge for a gallop down the wing and he sent Elliot Daly flying backwards with a barnstorming run before passing for Theo Chabouni to finish a superb move.
Another sweeping move fired the visitors into a 29-24 lead as Santiago Arata beat Saracens' defence to enable Seguret to score his second.
Saracens still had time to regain the lead but never looked like getting the score they needed, and the visitors were able to celebrate a stunning away win.
Saracens: Daly; Elliott, Lozowski, Tompkins, Segun; Burke, Van Zyl; Mawi, Dan, Balmain, Itoje (capt), Tizard; Gonzalez, Earl, Willis.
Replacements: George, Brantingham, Riccioni, Michelow, Knight, Simpson, Hartley, Hall.
Castres: Chabouni; Hulleu, Seguret (capt), Cocagi, Raisuqe; Le Brun, Doubrere; Guerois-Galisson, Colonna, Corato, Maravat, Jedrasiak, Moore-Aiono, Meka, Tukino.
Replacements: Zarantonello, De Benedittis, Azar, Macurdy, Peysson, Perrone, Dupont, Zeghdar
Referee: Andrea Piardi (Ita)