Former All Black star Sonny Bill William's Toronto Wolfpack have suspended training and stood down their entire UK-based staff, after four players experienced symptoms of coronavirus.
The Canadian club's action means that their next fixture against Wakefield next weekend will almost certainly be postponed, signalling a potential shutdown of the league.
The names of the four players affected haven't been revealed.
"Today, we have advised four of our players who have experienced mild possible symptoms of COVID-19 to self-isolate in accordance with the guidelines set by the UK Government," head coach Brian McDermott said.
"We take this very seriously and as a precaution, we have stood down our entire playing staff in the UK."
Williams is playing his debut season for the Wolfpack, after signing a two-year deal reportedly worth $10m, when his contract with NZ Rugby expired after last year's World Cup in Japan.
Toronto say they are now awaiting a directive from the Rugby Football League, who have instructed clubs to continue playing matches, but that could all change at a meeting of Super League clubs in Huddersfield on Tuesday (NZ time).
"We train in an environment where we interact with students and members of the public, and we vehemently do not want to act as a possible vector for COVID-19, which is why we have taken this immediate action," the Wolfpack announced.
"The health and welfare of our players, staff, fans and partners is always our primary concern.
"Our main goal is to prevent any further spread to our playing squad and their immediate family members, therefore we are now postponing all practice and have tasked our squad with keeping themselves healthy."