The club source said Messi, who has spent his entire career at Barca, informed the club about his desire to leave by sending a burofax, a service used in Spain to urgently dispatch a document that requires proof to third parties.
The 33-year-old Argentine, who joined the club in 2000 as a youth player, has a contract with the Catalans until 2021.
Barcelona confirm Ronald Koeman appointment after Quique Setien sacking
