Messi formally notified Barca on Tuesday that he intended to leave the club this summer using a clause in his contract that would allow him to leave at the end of the season without his €700 million release clause being paid if he notified the club within a specific time limit.
While Barca believe that period had expired in June, Messi's legal team argue that the clause remained valid until this week due to the delayed end to the campaign.
There is a feeling that attending training sessions ahead of the 2020-21 season could have a legal impact on his claim that he has already unilaterally ended his contractual relationship with Barca.
Messi has been assessing the situation with his advisors and informed the club on Saturday afternoon that he would not undergo coronavirus testing alongside his teammates.
Players who were involved in Barcelona's Champions League campaign this month -- including their disastrous 8-2 quarterfinal defeat to Bayern Munich -- are due to begin working with new coach Ronald Koeman on Monday.
Messi's refusal to turn up for testing and training raises the prospect of an awkward, ongoing stand-off between player and club.
Messi has not yet spoken out in public, although sources have told ESPN he is expected to do so at some point.
On Thursday, sources told ESPN that Barca president Josep Maria Bartomeu was willing to resign his position if it meant the player would change his mind.
Messi's decision to leave, however, is understood to be firm, with Manchester City favourites to sign him.
Sources have told ESPN that City are are confident Messi would snub any other team's effort to sign him.
His desire is to end his relationship with Barca in the best way possible, and on Friday he and his lawyers requested a meeting to begin talks over his departure.
The club, however, insist that he has a contract until 2021 and the only way to bring it to an end is to pay his €700m release clause.