THE first indication of how determined Lionel Messi is to leave Barcelona and how ugly this dispute could become will be gauged at the club's training ground on Sunday.

Barca's greatest ever player is due at the Ciutat Esportiva on Sunday morning for coronavirus tests, which must be taken and passed if he is to join Ronald Koeman's first pre-season session on Monday.

Messi's attendance or absence will be his first public demonstration since the burofax that stated his intention to leave for free arrived in the offices of Camp Nou on Tuesday evening and shocked the world of football.

Barcelona's newly-appointed sporting director Ramon Planes said on Wednesday the expectation was Messi would be present on Sunday for testing and then training on Monday as usual.

But while Messi was initially inclined to comply and avoid escalating tensions further, his lawyers have since advised him to stay away, Catalunya Radio were reporting on Saturday.

Taking part in the team's pre-season programme ahead of La Liga starting on September 12 could damage his legal case if his departure ends up being settled in court.

Barcelona insist a clause included in Messi's contract that allowed him to go for free this summer had to be activated by June 10 and is therefore no longer valid.

The only way Messi will be allowed to depart, the club argue, is through his release clause, set at 700 million euros ($833 million) until his contract expires on June 30, 2021.

But Messi's side believe the clause refers not to a specific date but to 10 days after the end of the season, which this year was extended into August due to the pandemic, with the Champions League final only played last weekend.

If the Argentinian refuses to show up for tests on Sunday, and by extension pre-season training on Monday, it will suggest there is no going back.

The assumption will be that Messi is prepared to do whatever it takes to quit the club he joined as 13-year-old boy, and where most expected him to continue all the way through to retirement.

- Barca standing firm, for now -

Instead, bitterness and resentment could accompany the unthinkable process of Messi being punished by Barcelona for absence, in the form of fines or a reduction in salary.

If he returns, those supporters who stormed Camp Nou in protest this week, may feel an inkling of hope.

Cajoled by team-mates -- although many of them expect to leave too Messi could be persuaded to stay.

Mundo Deportivo, the Barcelona daily with close links to the board, wrote on Saturday how the club is adamant the Argentinian will remain.

"The more the days go by, the firmer the board and president Josep Maria Bartomeu are in not moving from their position," it read. "Which is not to sell and, if possible, to renew the contract of Messi."

Yet every stance is arguably now a negotiating ploy and Messi's return on Sunday might only offer false hope to the fans.

Bartomeu made it known through the Spanish press on Thursday that he would resign if Messi said publicly that he was the problem and agreed to change his mind.

But Messi has instead asked for a meeting, not to negotiate, but to begin facilitating an amicable and dignified departure.

"Messi's decision to leave Barca irreversible," read Diario Sport on Saturday.

Manchester City remain favourites to sign the 33-year-old, whose basic salary alone amounts to around 60 million euros.

If Barcelona and Messi decide to avoid a lengthy legal process, a compromise selling price of around 100 million euros could be within reach for City, while eliminating their less wealthy rivals.

Messi has reportedly spoken to City coach Pep Guardiola already but others remain in the running, including Paris Saint-Germain, Inter Milan and Juventus.

The Italian sides would likely require a much-reduced price to afford any deal. But Juve might view the chance to combine Messi and Cristiano Ronaldo as an historic sporting and commercial opportunity too good to pass up.

All of them will be watching eagerly on Sunday.