England number eight Billy Vunipola is likely to miss the Six Nations after breaking his arm during Saracens' win over Racing 92 on Sunday.
It is the fourth such injury for Vunipola, 27, in two years - he has previously broken his right arm twice and his left once.
"He will see a consultant over the coming days to determine the full extent of the injury," Saracens said.
England boss Eddie Jones names his Six Nations squad on Monday.
England open their campaign against France on 2 February, with the tournament running until 14 March.
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Vunipola, who has 51 international caps, played every game in England's run to the World Cup final.
His possible absence boosts Harlequins' back row Alex Dombrandt's chances of an England call-up, while Saracens' Ben Earl, Sale's Ben Curry and Exeter's Sam Simmonds have also been impressing in the Premiership and Europe.