Why are Spanish football stars in legal trouble?

Spain has attracted arguably the three brightest lights of world football, with Cristiano Ronaldo of Real Madrid and Barcelona's Lionel Messi and Neymar all plying their skills in La Liga.

Over the past year, football fans have become used to seeing the trio caught up in accusations of tax fraud and other financial crimes by the Spanish courts.

And they are not the only players in the crosshairs of the Spanish judiciary. In 2016, Lionel Messi's Argentina and Barcelona team-mate, Javier Mascherano, received a one-year suspended prison sentence for tax fraud.

Lionel Messi and father Jorge were last year convicted of defrauding the Spanish state of €4.1m (£3.6m; $4.6m) in unpaid taxes on the striker's image rights, controlled by offshore companies in Belize and Uruguay.

The pair were both handed 21-month jail terms in a ruling recently confirmed by Spain's supreme court.

Now the original Barcelona trial court must decide whether the sentences should be suspended in accordance with Spanish custom for first-time offenders whose prison terms do not exceed two years.

Prosecutors have asked for a two-year sentence and a €10m fine for Neymar, who was cleared of fraud but ordered to stand trial over alleged corruption in his 2013 move from Brazilian club Santos to Barcelona.

Now Ronaldo has become the third and final member of the elite La Liga trio to face criminal accusations, after prosecutors announced they were pursuing the 32-year-old former Manchester United man on four counts of tax fraud.

A source close to Ronaldo told the BBC that "he's very sad and really upset" about the allegations. "He doesn't want to stay in Spain. At this moment, he wants to leave," the source said.

Soon after David Beckham joined Real Madrid in 2003, he was able to enjoy a new tax-exemption scheme aimed at attracting foreign talent to Spain across all sectors. That scheme became known as the Beckham Law, when he became one of the first players to sign up to a six-year-long tax ceiling of 24%, roughly half what Spaniards paid on six-figure-plus incomes.

Spain was in the midst of an unprecedented economic boom, a perfect playground for "galacticos" of the likes of Zinedine Zidane and Luis Figo, before the arrival of Cristiano Ronaldo and the emergence of Barcelona prodigy Lionel Messi.

But in 2010 the Beckham Law was scrapped for salaries of more than €600,000, and since then tax inspectors have begun to wise up to the use of complex financial operations using offshore shell companies to get around tax laws.

"The line between avoidance and evasion is very fine in these cases. In the past few years Spain's tax agency has intensified its control over footballers and their companies, checking to see if they are mere fronts or whether they are really active economically," explains Carlos Cruzado, president of tax inspectors' union Gestha.

Neymar is the odd one out. His case involves alleged wrongdoing towards a contractual party regarding his transfer fee, but the forward has been found guilty in his native Brazil for tax fraud on money earned while playing for Santos.

The Messi and Ronaldo cases are similar. Both are accused of avoiding tax on sale of image rights by using offshore companies. However, the Portuguese was registered as a non-resident taxpayer under the Beckham Law, while the Argentine has spent his entire adult life registered in Spain.

Prosecutors accuse the Real Madrid star of evading tax of €14.7m between 2011 and 2014 via an alleged shell company called Tollin Associates, registered in the British Virgin Islands.

Spanish investigators say the company, owned by Ronaldo, is a "screen" and has no economic activity apart from having bought and then ceded the player's image rights to a firm based in Ireland that "genuinely manages [his] rights sales".

Prosecutors also claim that money earned from image rights was incorrectly described as capital gains, to benefit from a lower tax rate.

Lionel Messi was informed by the judge in his case it was no defence to plead ignorance and argue that his father was the only person who knew how his money was being managed.

Neymar has denied any wrongdoing and told the court investigating his case that his father and associates dealt with off-field business matters.

Cristiano Ronaldo's representatives and legal team say the only dispute can be about quantity and that there has been no intention to commit fraud. "There is no tax evasion scheme... There has never been any hiding nor any intention to hide anything," they say.

They argue he has paid tax to the Spanish treasury on 20% of his total image rights when, in fact, more than 90% of these are generated outside Spain as he is such a global name.

"The tax agency clearly thinks that if he is being paid for wearing certain boots, shirts or caps in Spain, then he cannot claim this money is being earned abroad," explains Mr Cruzado.

Neymar and Lionel Messi look set to be spared prison due to Spain's unwritten two-year-sentence rule, even if Neymar is eventually found guilty.

Cristiano Ronaldo may be a different matter. Three of the four accusations of tax fraud are considered by prosecutors to be "aggravated", so they carry a minimum sentence of two years each. Four guilty verdicts and he could face as many as seven years.

However, an investigating judge needs to ratify the prosecutors' accusations, and that could take many months or even years.

Even if the investigating magistrate does take up the case, the Portuguese will have several options and a guilty verdict would not necessarily mean jail.

He could admit guilt, pay taxes and fines in advance and reduce any eventual jail term to a half or quarter of the statutory minimum. That way he would slip under the standard two-year bar for first-time offenders and see his sentence suspended.