Lionel Messi has told Barcelona he wants to leave the club on a free transfer – after nearly two decades with the Catalan giants.

The club has confirmed that the Argentina great sent a fax expressing his desire to leave.

The announcement comes 11 days after Barcelona’s humiliating 8-2 loss to Bayern Munich in the Champions League quarter-finals, one of the worst defeats in the player’s career and in the club’s history.

The defeat capped a difficult season for Barcelona – the first without a title since 2007-08 – and ignited one of its worst crises.

Messi, 33, has won a record six Ballon d’Or awards during his time at Barcelona as the top player in the world, and has helped the club win 10 Spanish league titles and four Champions League crowns.

However, his attempt to exit the club could lead to a legal battle over a clause in his contract which Messi’s camp believes entitles him to walk away with no fee.

Barcelona acknowledged the clause but said the deadline for activiating it expired in June.

Messi’s contract also includes a 700 million euro ($A1.2 billion) buyout clause.

If Messi can leave for free, he will have no shortage of suitors.

However, only a handful of clubs could realistically afford to pay his wages even without a transfer fee – with mega-rich Paris Saint-Germain and Manchester City widely seen as perhaps the most likely destinations.

Former Barcelona captain Carles Puyol has tweeted his support for Messi, saying “respect and admiration, Leo. All my support, friend”.

Tweet from @Carles5puyol

Messi had been outspoken about his dissatisfaction with some of Barcelona’s decisions this season.

He had been silent since the loss to Bayern, though, raising doubts about his future.

-with agencies