England captain Owen Farrell has tested positive for Covid-19 on the eve of this weekend's match against Tonga at Twickenham, to the Rugby Football Union (RFU) said on Friday.
A statement from the English game's governing body said Farrell would miss a training session later in the day and remain in isolation before undergoing further tests.
A spokeswoman for the RFU said it was too soon to say if Farrell had been ruled out of Saturday's Test against the Pacific Islanders, although it would appear the fly-half's place for England's opening match of their November campaign is in severe doubt.
All other England players and staff had a round of PCR testing on Thursday after a member of head coach Eddie Jones' backroom team returned a positive result, with Farrell the only other confirmed case.
It would appear that the best hope for Farrell, on the verge of making his 100th Test appearance following 93 caps for England and six for the British and Irish Lions, is that his sample turns out to be a false positive.
Losing their skipper and a senior player would create further disruption to England's back-line, with Jones' hopes of playing Marcus Smith at fly-half alongside Farrell at inside centre scuppered by a leg injury that has left the Harlequins rising star on the bench.