Portuguese champions Sporting Lisbon are one of eight clubs who have been sanctioned by Uefa for Financial Fair Play (FFP) breaches.
Sporting have been fined 250,000 euros (£213,694), while league rivals Porto were fined 300,000 euros (£256,462).
They have until 31 January 2022 to pay the overdue amounts otherwise face a ban from Uefa competitions.
Sporting have qualified for the knockout stage of this season's Champions League.
The other six clubs sanctioned are Spain's Real Betis, Astana of Kazakhstan, Romania's CFR Cluj, CSKA Sofia of Bulgaria, Gibraltar's Mons Calpe and Portugal's Santa Clara.
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Uefa said all eight clubs were sanctioned due to their "non-compliance with the 'no overdue payables' requirement".
A statement from European football's governing body warned they faced additional punishment if they did not pay the overdue amounts.
"A conditional exclusion from participating in the next Uefa club competition for which they would otherwise qualify in the next three seasons... unless they can prove by 31 January 2022 that they have paid the outstanding overdue amounts," added the statement.