Saracens’ European Challenge Cup game with Pau on Saturday has been called off after positive Covid-19 tests at the English club.
The former European champions say they have experienced an outbreak “within the training ground environment”.
Saracens have shut the training ground and all players, coaches and support staff have gone into isolation.
Pau will be given a 28-0 win and five points in Pool C as a result of the decision to cancel the game.
Saracens have not detailed the number of positive tests but say “several positive cases” emerged after a round of PCR testing that took place on Monday.
“We have been working closely with EPCR [European Professional Club Rugby] and PRL [Premiership Rugby Limited] and it is with regret that our game versus Pau has had to be forfeited due to our inability to field a 23-man squad for this fixture,” a club statement read.
“This is extremely disappointing for everyone at the club but as always the health and wellbeing of our people is our priority at this time.
“The squad will return to full training on 24 December following the necessary period of isolation.
“We would like to reassure our supporters that our Gallagher Premiership fixture versus Worcester Warriors on 26 December will still take place,” the statement added.
It is the second game in European rugby to be called off this season as a result of Covid-19.
Scarlets’ clash with Bristol in the Champions Cup was cancelled after the club had travelled and quarantine issues returning from their United Rugby Championship games in South Africa after some Covid-19 cases.
Meanwhile, Cardiff Blues were forced to put out a makeshift side against Toulouse, with 32 players forced to quarantine upon their return from South Africa, as well as a further 10 out because of injury and suspension.
It is the first time this season an English side has had to call off a game in any competition due to Covid-19.
In football, Manchester United were forced to postpone their Premier League trip to Brentford on Tuesday after an outbreak at the training ground, while Tottenham were unable to play their Europa Conference League game with Rennes and their Premier League game with Brighton for the same reason.
It comes as coronavirus cases have begun to rise, with the Omicron variant becoming more prevalent in the UK.