UEFA wants its sponsors, guests and media representatives to be excepted from quarantine restrictions - otherwise, the all-English final could be played in Portugal.
UEFA has given the British government until Tuesday to agree to travel waivers for sponsors, guests, and media that it wants to fly into the UK for the Champions League final if it is held at Wembley rather than in Istanbul.
If it does not agree to the exceptions, UEFA has warned, the final could be played in Portugal instead.
The May 29 showpiece between Chelsea and Manchester City is set to be moved from Istanbul after Turkey was added to England's "red list" of countries with severe coronavirus outbreaks, presenting a challenge for travelling fans.
Portugal is on England's "green list" of 12 low-risk territories where people are not required to quarantine upon their return from next Monday, making it easier for thousands of fans.
Porto is one of the backup options after UEFA hit snags with its preferred location of London.
UEFA held talks with the British government on Monday but was unable to secure exemptions from quarantine for everyone they want to be able to attend a final in London, including sponsors of the Champions League, their guests, and media, according to sources close to the talks. They spoke on condition of anonymity because they were not authorised to discuss the talks.
UEFA has given the British government until Tuesday to decide whether authorities can offer all of the travel waivers required.
Switzerland, where UEFA is based, is among the amber list of countries from which people arriving in England have to quarantine for 10 days, albeit organised individually rather than via the mandatory hotel isolation in place for arrivals from red-list countries.