A non-playing member of the St Helens staff has tested positive for Covid-19, the Betfred Super League club have announced.
Saints have cancelled their planned training activity for Wednesday and the unnamed individual, who is asymptomatic, will be retested.
Castleford, Saints' opponents last weekend, have been informed but the Merseyside club have said there is no need for any Tigers staff to isolate "under the test and trace procedures agreed for the resumption of rugby league with Public Health England".
Neither Saints nor Castleford have a game this weekend and no fixtures have yet been postponed.
A statement from Saints read: "The early results from this week's routine testing for Covid-19 across the 11 Betfred Super League clubs have included one positive test, for a non-playing member of our staff.
"The club have postponed our planned training activity for Wednesday as we work with the RFL on test and trace analysis relating to the relevant individual.
"There will also be a retest of the staff member who has tested positive, but who is not displaying any symptoms – with the possibility of further testing for the squad this week as a precaution."
Castleford tweeted: "We would like to wish the member of staff at @Saints1890 well, furthermore, following the latest round of tests on Monday, everyone at Castleford Tigers has received negative results."
The latest positive test comes after 12 staff at Hull, including nine players, tested positive earlier this month.
Hull are now in isolation and had last weekend's Super League game against Castleford as well as this week's Challenge Cup sixth-round tie – also against the Tigers – postponed.
Salford, who were Hull's last opponents on August 9, have also been isolating although all tests carried out at that club have been negative. Salford's game against Catalans last weekend was called off.
Saints are next due to play Hull KR at the Halliwell Jones Stadium on August 30.