The Pro14 game between Irish sides Munster and Leinster on 26 December has been postponed in accordance with coronavirus guidelines.
The inter-provincial derby was scheduled to be played at Thomond Park in Limerick.
Champions Leinster are “investigating the outcomes of some of the results” from the latest round of Covid-19 testing.
The Pro14 is exploring potential dates to reschedule the game “in early 2021”.
“Results of these investigations will not be returned in time to meet guidelines set by public health authorities in order for the game go ahead as planned,” read a statement from both provinces.
“Having considered this, the Pro14 Rugby Medical Advisory Group has agreed that this fixture cannot go ahead as scheduled.”