The European Champions Cup and Challenge Cup have been temporarily suspended after the French authorities ruled the participation of Top 14 clubs to be too much of a public health risk.
European Professional Club Rugby (EPCR) made the announcement on Monday following a meeting last week with French government officials.
Due to concern over a new variant of coronavirus, the French government has decided to prevent its clubs from participating in games both in France and in the United Kingdom and Ireland.
It means the EPCR has been forced to suspend the Champions Cup with two rounds of the pool phase remaining. The Challenge Cup is in its preliminary phase.
The development would appear to place the 2021 Six Nations, scheduled to start with a meeting between France and Italy in Rome on February 6, in jeopardy.
Last year's Six Nations tournament was only completed in October because of the pandemic-enforced delay.
An EPCR statement read: "EPCR and the Ligue Nationale de Rugby took part in a meeting by audio conference last week with representatives of the French ministries of the Interior, Health and Sport, as well as of the President's office.
"Notable updates to EPCR's COVID-19 protocols were presented to the French authorities, including the addition of PCR tests no earlier than three days before matches in the tournaments, conforming to French government guidelines and supplementing the existing, meticulous contact tracing measures put in place with a view to limiting the risk of transmission.
"Against the backdrop of the recent detection of a new strain of coronavirus, the French government has directed that French clubs postpone their participation in EPCR's tournaments for the month of January, both for matches scheduled in France and for those due to be played in the United Kingdom and Ireland.
"On the basis of this directive, EPCR had no choice but to temporarily suspend the pool stage of the Heineken Champions Cup and the preliminary stage of the Challenge Cup.
"While respecting all further directives by governments and local authorities, and prioritising the health and welfare of players and club staff, EPCR, in conjunction with its shareholder leagues and unions, remains committed to trying to find a solution which will enable it to resume and complete the tournaments as soon as practicable.
"EPCR will be making no further comment at the present time."