The venue for the Champions League final is expected to be confirmed on Thursday with Porto now the clear favourite, the PA news agency understands.
The match was originally due to be played in Istanbul but UEFA has been seeking an alternative since the British Government placed Turkey on its high-risk 'red list' for international travel amid the coronavirus pandemic.
The European governing body has been involved in talks with UK authorities this week over the possibility of switching the game to London but an agreement has not been reached.
It is understood the main sticking point in negotiations has been over the number of quarantine exemptions UEFA has sought for sponsors, VIPs and broadcasters. The Government has so far been unable to agree to UEFA's requests.
Amid the discussions, Portugal emerged as a potential alternative. Multiple sources have told PA that Porto is now in pole position if UEFA receives clarity on entry restrictions and stadium capacity limits at the preferred venue of the Estadio do Dragao.
UEFA wants to know what will happen when current international travel restrictions in Portugal end on Sunday. Its hope is that rules can be relaxed to allow fans to travel from England to Portugal.
Currently travel into Portugal from countries outside the European Union is only allowed in limited essential cases.
Fans would not face much hindrance returning to the UK, with Portugal currently on the Government's 'green list', meaning quarantine is not required.
UEFA will also want to know how many spectators can attend the match. The Portuguese top flight announced on Wednesday that fans will be allowed to attend the final round of matches next week, but venues will be limited to 10 per cent of capacity.
UEFA will clearly hope the Portuguese authorities will be prepared to allow a higher capacity for the final.
If satisfactory guarantees are received, a decision could be confirmed on Thursday.
Both clubs are familiar with the Estadio do Dragao having played Porto during this season's competition.
It would also be the second successive year that Portugal has stepped in to host the latter stages of the Champions League. In 2020, Lisbon's two main stadiums hosted all matches from the quarter-finals onwards in a rearranged mini-tournament.
One issue already confirmed is the team of officials for the final. They are from Spain, with Antonio Miguel Mateu Lahoz chosen to referee the contest.