Barcelona may be forced to spend millions of euros making Lionel Messi their highest-paid player when the full details of Neymar's transfer and contract come to light, according to reports in Spain.

Barca president Sandro Rosell maintains the club paid 'just' €57.1 million to sign Neymar from Santos last summer. However he has not specifically denied claims in El Mundo that Neymar and his father/agent received more than €60 million in various commission and signing-on fees between them.

The deal is currently under investigation by a Spanish judge and AS reports that, should details emerge that Neymar is earning significantly more than meets the eye, a number of Barcelona players will look for improvements to their contracts, with four-time World Player of the Year Messi likely to be at the front of the queue.

Neymar's basic salary is thought to be €6 million a year, with bonuses for winning trophies and meeting certain individual targets. If the extra €60 million he reportedly earns is spread over the five years of his contract, he would take home more than €18 million a year - well above Messi's reported €11 million.

Cristiano Ronaldo is now believed to earn €20 million since signing his new Real Madrid contract last September.

Messi - set to make his 400th Barca appearance in Wednesday's Copa del Rey quarter-final first leg at Levante - last extended his contract in February 2013, committing to Camp Nou until June 2018, in what was his sixth renewal in eight years.