The announcement by European football's governing body came after a videoconference meeting of its Executive Committee.
UEFA on Thursday left the door open for leagues to bring an early end to their seasons during the coronavirus pandemic but said qualification for the next Champions League should be "based on sporting merit".
The announcement by European football's governing body came after a videoconference meeting of its Executive Committee and with several countries awaiting approval to end their seasons because of the complications brought about by imposed virus lockdowns.
In a statement, UEFA said completing seasons in their original formats remained "the ideal scenario, should the pandemic situation permit it." If not, leagues were urged to find ways of restarting "with a different format", which could mean play-offs being introduced in some cases.
"If a domestic competition is prematurely terminated for legitimate reasons...UEFA would require (leagues) to select clubs for the UEFA club competitions 2020/21 based on sporting merit" in the current season.
That would appear to definitively rule out any possibility of seasons being voided altogether. Instead, national associations are left with the choice of trying to restart somehow or, if needed, calling an end to the season with current tables considered final and Champions League and Europa League places dished out on that basis.
The "legitimate reasons" outlined by UEFA for ending seasons now included leagues being prevented from doing so by governments "before a date that would make it possible to complete the current season in good time".
Earlier this week the Dutch football federation (KNVB) announced its intention to call an end to the season following a government decision to extend a ban on large gatherings until September because of the pandemic, which has caused more than 110,000 deaths across Europe.
Ending the Eredivisie season now would mean Ajax being declared champions although they are level on points with second-placed AZ Alkmaar with nine games still to play. AZ have won both league meetings with Ajax this season.
The Belgian Pro League is hoping to confirm next week that it will end its season, thereby declaring Club Brugge champions.
The Scottish league could do the same after a controversial resolution passed last week by the 42 member clubs to award positions on a points-per-game basis. The lower leagues have already been declared over and doing the same for the Premiership would mean Celtic being handed a ninth successive title with eight games still to play.
In contrast, the German Bundesliga is hoping to restart early next month, with games behind closed doors.
It remains to be seen if Europe's other leading leagues -- halted since mid-March because of the health crisis -- will be able to do the same.
There are still 11 rounds of matches left in Spain's La Liga, for example, yet UEFA remain keen to get the 2019-20 season over and done with by the end of August. That would mean fewer delays in starting next season.
UEFA's own calendar working group has been working on two potential scenarios, with one seeing domestic and European competitions restarting in parallel and the other seeing domestic leagues being played out before the Champions League and Europa League are completed in August.
In the latter scenario, the Champions League final would be pencilled in for August 29, with the Europa League final two days earlier. Both competitions are currently frozen in the last-16 stage, but any resumption depends on travel restrictions being lifted.