Toulon pulled out of their European Champions Cup match against Scarlets, scheduled for Friday, because coronavirus "sanitary conditions were not met", despite organisers being "satisfied" it was safe to play the game.
The match was set to be the first of this weekend's second round of matches.
"The Rugby Club Toulonais learned this Thursday, December 17, that at least one player from Scarlets had tested positive for Covid-19 at the start of the week," said the Top 14 club.
"Although aware of the serious consequences, we decided not to play this match as planned this (Friday) evening."
This is the first voluntary forfeit by a club in the competition.
Earlier in the week, tournament organisers EPCR cancelled three other games in this round -- Toulouse v Exeter, Glasgow v Lyon and La Rochelle v Bath -- because of Covid-19 cases at the British teams.
On Friday evening, EPCR confirmed that the French teams in those three matches had been awarded 28-0 victories.
EPCR said the Toulon match had been postponed.
It said the Scarlets player in question self-isolated and that two others were left out of the Welsh team's squad.
"The player self-isolated in accordance with public health guidelines and his two close contacts were not selected in the matchday squad," EPCR said in a statement.
"A medical risk assessment committee was convened earlier today (Friday) to consider any potential issues in relation to Scarlets' testing and tracing, and it was satisfied that the risk of further transmission had been contained before recommending that the match could go ahead safely."
Toulon were also given the option of playing later in the weekend instead, but turned down the offer.
"It was suggested that the match could be postponed to a later date in the weekend to allow any of the club's players to opt out of the match if they wished to do so," EPCR added.
"It was also suggested that RC Toulon could source willing players from their existing tournament squad to replace those who had withdrawn and that the Scarlets matchday squad could undergo additional PCR testing.
"These offers were rejected by RC Toulon, and EPCR can therefore confirm that the match is postponed."
Both Toulon and Scarlets won their opening matches last weekend.