Bordeaux-Begles reached a European semi-final for the first time after beating visiting Edinburgh 23-14 as Castres were disqualified on Saturday in the Challenge Cup.
Leicester Tigers advanced to the last four after the French club, who recorded four Covid-19 cases, were deemed to have forfeited the match scheduled at Welford Road on Sunday.
"The decision followed a meeting of a Medical Committee comprising medical leads from the unions, league bodies and both clubs, and subsequent independent medical advice," European Professional Club Rugby (EPCR), who run the European club competitions, said in a statement.
"Castres have to forfeit the match and Leicester will qualify for the Challenge Cup semi-finals."
Next weekend, Bordeaux-Begles will meet another club playing in a European final four for the first time, Bristol, who thrashed the Dragons 56-17 on Friday.
Victory gives Bordeaux-Begles a chance to salvage something from a 2019-20 season that was going so well until the coronavirus pandemic ended the French Top 14 campaign in March when they were leading the league.
In front of 1,000 noisy fans at Stade Chaban-Delmas, the home side scored two tries in the opening eight minutes through Argentina's Santiago Cordero and Jean-Baptiste Dubie. France fly-half Mathieu Jalibert converted both.
"Even with 1,000 spectators, we heard them a lot," said captain Jefferson Poirot. "You have to tip your hat to them."
A Jalibert penalty put Bordeaux-Begles 17-6 up 50 minutes.
But playing only their second competitive match since club rugby resumed in France, coach Christophe Urios' team began to make mistakes and the Scottish visitors fought back with a try by winger Damien Hoyland.
With 10 minutes to go, the hosts were reduced to 14 men when Tonga prop Ben Tameifuna earned a yellow card and Blair Kinghorn cut the lead to six points from the subsequent penalty.
But the home defence held and Ben Botica sealed victory with a last-minute penalty.
"Physically, they are more ready than us," Jalibert said. "We started the match with a big lead, we caught them by the throat at the start and little by little we lost momentum, we started to make mistakes and they came back."
On Friday, Dragons jumped to a 10-0 lead at Ashton Gate but Bristol responded with seven tries, two by Ben Earl, to win 56-17 and earn Fiji centre Semi Radradra a rematch with the club he left over the summer