Cristiano Ronaldo signed a new five-year deal with Real Madrid in November that helped him become the best-paid athlete in the world.
But now a source close to the player now says he "doesn't want to stay in Spain" because he is "upset" after being accused of tax fraud in the country.
Ronaldo, 32, joined Madrid from Manchester United for a then world-record £80m in 2009.
The Portugal legend has scored a club-record 406 goals in his 394 appearances for Real.
Only two weeks ago, he helped Zinedine Zidane's side win a 12th European Cup with two goals in a 4-1 victory over Juventus in the Champions League final in Cardiff.
Will that turn out to be his last appearance for Los Merengues? Where could he go? And what would Madrid do without him?
Cristiano Ronaldo scores against Hungary
June 2016: Ronaldo's Euro 2016 campaign did not start well - 20 shots, zero goals for Portugal. But a superb, flicked finish against Hungary in the group stages made him the first player to score in four different European Championship finals.
Maybe he's just upset. Maybe it is something he said in a moment when he's really angry. We don't know yet. He's been happy here, he helped us to win three Champions Leagues over the past four years, so I hope he will stay.
But, if he's made his mind up, I don't think it's going to be easy to change. He's very strong-minded and clear in what he wants. If he leaves I don't see him in a place like China or the Middle East. He will be always playing for a big club and trying to get more titles and more personal awards.
He's a fighter and he's in good shape. He can play again for the best club and I think that club now is Real Madrid. I can't tell you what he's doing but I hope he will stay, I'm sure he'll stay.
'His people called all the Spanish papers'
Richard Martin, sports correspondent in Spain for Reuters
This is not the first time Cristiano Ronaldo has sought to demonstrate his power at Real Madrid by flirting with leaving. In 2012 he used a pitch-side interview to declare "I'm sad and the club know why," provoking panic among fans and at boardroom level.
This latest move, using a newspaper from his own country to declare his discontent, feels remarkably similar. It should also be noted that sources close to Ronaldo called the offices of all Spanish newspapers last night to inform them of the impending front cover story in A Bola.
'Ronaldo wants to leave Spain' was the headline on Friday's A Bola - a Portuguese football paper
This latest power struggle is likely to have the same outcome as the last: the offer of an improved contract to legislate for the damage caused by the tax scandal. There is also more than a whiff of deflecting attention from his wrongdoing to a soap opera of where his future lies, emphatically shifting the agenda.
Although he is their all-time top scorer, Ronaldo does not feel undying loyalty to Madrid and has repeatedly aired his grievances at being occasionally booed by supporters.
Even so, the four-time world player of the year believes he still has plenty to give at his 32 years and he knows there is no better club for him than the European and Spanish champions, which is why, in reality, he is going nowhere.
'Madrid won't stand in his way - they want Mbappe'
Pete Jenson, Madrid-based football writer
Ronaldo's intention of leaving Real Madrid may well come to nothing, but if he does not back down and looks to force his way out of the club, Madrid will not stand in his way providing they find a club prepared to make it worth their while.
The reaction in Madrid on Friday was one of calm acceptance. The player has indicated a desire to leave before, most notably in 2012 when he complained of not being supported by the club's directors.
Cristiano Ronaldo won the first of his four Champions League trophies at Manchester United in 2008 after they beat Chelsea in Moscow
The question will be: who buys him? His 1bn euro (£874.88m) buyout clause is beyond everyone's reach but Madrid would probably accept closer to 150m euros (£131m) and immediately look to reinvest it in 18-year-old Monaco forward Kylian Mbappe.
His former club Manchester United would be the romantics' choice to take him but Paris St-Germain are perhaps more likely suitors.
They have long been admirers and their president Nasser Al-Khelaifi is a friend of the Portugal international. Ronaldo would fill the hole left 12 months ago by Zlatan Ibrahimovic.
Who could afford him?
Rob Wilson, football finance expert and lecturer at Sheffield Hallam University
Manchester United, Manchester City, Arsenal, Chelsea, Bayern Munich and Juventus would have the financial resources to put together a competitive offer within the realms of Financial Fair Play rules.
The other side of the question would be whether they would want to commit so much on one player.
It would be interesting to see what kind of fee Madrid would demand. I would estimate something in the region of £100-120m. To put that into a wider context, that's like a Premier League club spending the entirety of its TV deal money on one player. Plus of course, there's the wages on top of that.
People cannot criticise - Ronaldo
Another point to bear in mind is that this would not be a 'Galactico' move, where the value of commercial endorsements and merchandise sales would help a club recoup money spent on the transfer fee.
Ronaldo comes surrounded by a group of very well-informed advisors, who already have all various commercial deals in place. For a potential buyer like Manchester United, they might look to leverage something from the story of his return to the club, somehow make that commercially attractive, but we are scratching around on the margins really.
Where you can start to make leverage on players is when they are younger - which is why a player like 18-year-old Monaco striker Kylian Mbappe would be a far more attractive proposition. A buying club knows they can make an awful lot back.
Manchester United and all of their fans would love to have Ronaldo back. He made 292 appearances in his time at the club, scoring 118 goals. Wayne Rooney has played double the number of games and scored double the number of goals, but he is held in nowhere near as high esteem.
The fans still sing Ronaldo's name, and all the talk on social media today has been encouraging United chief executive Ed Woodward to get him back.
The problem for United is that they have been down this road before. They have thought they could get players from Madrid in the past, notably defender Sergio Ramos in 2015, who ended up signing a new deal with the Spanish club.
United will be surveying the situation, keeping their head down. They won't be telling anyone that yes, they want to sign Ronaldo, but if they got the chance they would be straight in there.
The word is that he and Jose Mourinho fell out when the latter was in charge of Madrid, but if you were to ask United fans to choose between the two, they would say Ronaldo.
It would be up to Jose to make it work, because I don't think many United fans would understand if they did not take the chance to sign him - if the option was there.
Ronaldo is only one of four players in the club's history to have won the Ballon d'Or while still a United player. The other three - George Best, Dennis Law and Bobby Charlton - have a statue outside the stadium.
Archive: Ronaldo refuses to rule out Man Utd return in 2015
'You can't replace 50 goals a season'
Eduardo Alvarez, Madrid-based football writer speaking to BBC World Service Sport
The reaction among Madrid fans has been one of shock and frustration after a fantastic end to the season. If the Spanish taxman wants him to pay his taxes and he feels mistreated there is not a lot the club can do. You cannot replace 50 goals a season for the past nine seasons. It would be huge if he left Madrid.
Ronaldo has won the Ballon d'Or four times - most recently in 2016
It just seems the wrong moment to do this, especially to say it through a Portuguese paper. But it is obvious he feels unfairly treated. He can voice his concerns but the reaction of many supporters is that he should talk to the club instead of talking to the press.
It is unbelievable, most Madrid fans felt like they had a good five years of success ahead of them. The fact that the top scorer seems to be leaving suddenly, out of nowhere, no-one will have expected that.