Lionel Messi has told Barcelona that he wants to leave the club this summer, sources have confirmed to ESPN.

The club confirmed to ESPN Deportes' Jordi Blanco on Tuesday that they were notified by Messi of his decision via burofax, a service used in Spain to urgently dispatch a document that requires proof to third parties. The club responded to Messi via the same service saying they want him to stay and finish his career at the club.

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Messi, 33, has a contract until 2021 but has grown disappointed with events on and off the pitch at Camp Nou in recent months. Reports from Argentina claim Messi believes a clause in the contract should oblige Barcelona to let him leave for free. However, as reported by ESPN earlier this year, the club maintain that particular clause expired at the start of the summer.

The debated clause was included in the last contract signed by Messi, in 2017, and stipulated he could leave for nothing at the end of each season as long as he communicated his decision to Barca before June. Otherwise, any rival club wanting him would have to pay a €700 million release clause.

A source at Barcelona has told ESPN they firmly believe that Messi's window to leave for free this year has passed, but other reports say Messi's legal team are arguing the extension of the season altered the expiry date of the clause.

Barcelona's 8-2 Champions League semifinals humiliation last week at the hands of eventual tournament winners Bayern Munich proved the final straw, and Messi has now informed the club of his desire to embark on a new challenge following a first trophyless season since 2007-08.

Coach Quique Setien was sacked after the Bayern loss. Messi met with Setien's replacement, Ronald Koeman, last week, telling him that he had doubts about his future in Catalonia. After several days of further reflection, Messi has now decided he wants to spend the final years of his career away from Barca, who he first joined almost 20 years ago as a teenager.

On Monday, ESPN revealed that Manchester City are already looking into the feasibility of signing the Argentine forward. The Premier League side are crunching the numbers that would be involved in the deal to see if they would be able to make the signing without breaking financial fair play rules.

It remains to be seen what other clubs would be in contention as very few would be able to offer Messi what he wants on the pitch -- success in the Champions League -- and, at the same time, match his wages as the best-paid player in the world.

Messi, the six-time World Player of the Year, has considered leaving Barcelona before but has never previously taken his desire to leave to the club this far. He is conscious that at 33 he won't have many more opportunities to add to the four European Cups he has already won and has decided he will have a better chance of more silverware elsewhere after a string of disappointments with Barca.

The Blaugrana last won the Champions League in 2015, but since then have suffered a string of embarrassing exits. They gave up three-goal first-leg leads against AS Roma and Liverpool in 2018 and 2019, respectively, and were then thumped by Bayern in Lisbon last week. That has prompted plans for an overhaul of the squad at Barca which could include parting with Luis Suarez, too. Sources have confirmed to ESPN that Suarez, Messi's best friend at the club, will be allowed to leave this summer.

Meanwhile, Messi has also grown frustrated with the people in power at Barcelona. He had a public fallout with former sporting director Eric Abidal earlier this year and criticised the board for attempting to put pressure on the players when they were negotiating a wage cut due to the coronavirus pandemic.

Information from Reuters was used in this report.