Lionel Messi turned 26 on Monday having claimed every prize as a player bar the World Cup but facing new challenges that will test the Argentine's skills off the pitch as much as on it.

Barcelona's four-time World Player of the Year is dealing with a Spanish tax investigation after authorities accused him and his father Jorge of a four million Euros fraud on earnings from image rights from 2007-2009.

The Messis have been summoned to give evidence to an investigating judge on Sept 17, after which formal charges could be brought.

They could face up to four years in jail if found guilty, according to Spanish law.

The talismanic forward also has to handle the arrival of one of the sport's most exciting prospects at the Nou Camp, Brazil striker Neymar.

The 21-year-old has been signed by the Spanish champions for 57 million euros with huge media fanfare, and will be vying for attention with Messi on the field.

The Messis have denied any wrongdoing and appeared to have made a conciliatory move with the tax man, according to a report in Monday's edition of Barcelona-based newspaper La Vanguardia.

They said Messi had filed two supplementary tax returns last week for 2010 and 2011, paying in an extra 10 million euros for earnings related to image rights.

Media reports had suggested the Spanish tax authorities were considering extending the scope of their original complaint to cover these two years as well.

La Vanguardia said this voluntary additional payment was an indication that the Messis were seeking to reach an agreement.

The prosecutor's office said two weeks ago that Messi had tried to hide income from the sale of image rights via a complex web of shell companies in Uruguay, Belize, Switzerland and the United Kingdom.

Starting from next month, Messi and coach Tito Vilanova will also need to start working out how to blend Neymar into their side for next season.

"I'm sure they will fit together well and understand each other," Barca teammate Gerard Pique said recently. "The presence of Neymar will help Leo.

"Often rivals try to mark Messi and crowd him out, with a pile of opponents on top of him. With Neymar, at least two of them will need to be wary of someone else."

Messi came up through the Barca youth ranks and has adapted before to tactical changes, moving off the wing into a more central role, taken on more responsibility and continued to break records with dizzying regularity.

Theoretically, he has yet to reach his soccer prime but has won three Champions League crowns with Barca in 2006, 2009 and 2011.

He has six La Liga winners medals, two King's Cups, two European Super Cups, five Spanish Super Cups and two Club World Cups.

On an individual level he has topped the scoring charts in the Champions League, the most prestigious club competition in the world, on four occasions and holds the La Liga scoring record with 50 goals scored in the 2011-12 campaign.

He set a new record of 91 goals scored in a calendar year in December, and went on to become the first player to score against all of Barca's La Liga rivals in consecutive games with a run of 30 goals in 18 games during last season.

Neymar's arrival could either help or hinder Messi's progress at club level, but looming is the chance to improve his chances of being rated as the best player of all time.

Soccer greats Pele and Diego Maradona won the World Cup with Brazil and Argentina respectively, and around the time of his next birthday Messi is likely to be in the middle of trying to claim the one major prize that eludes him.

Winning the 2014 World Cup in Brazil would be another major achievement for a player challenging to be remembered as the greatest of all time.