The Argentina superstar sent a fax to club chiefs on Tuesday informing them he wants out and requesting they exercise a clause in his contract which would him to leave for free.
There are likely to be plenty of disputes about whether he can actually enact that clause because it expired in May.
Messi is set to argue that because of the coronavirus pandemic it should be extended.
Should he leave on a free transfer, then clubs will be scrabbling to sign the 33-year-old who has only played for Barcelona throughout his entire career.
The six-time Ballon d’Or winner will have offers from across Europe but there are only certain clubs who will likely be able to afford his wages.
Messi, who has spent his entire professional career at Barcelona, 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, who joined the club in 2000 as a youth player, has a contract with the Catalans until 2021.
If the deal goes ahead, it could cost Man City £500m.
Any club wanting to sign Messi will have to pay about £90m a year to match his wages at Barcelona.
Manchester City rivals, Manchester United have the money to buy Messi, but Borussia Dortmund and England winger Jadon Sancho, 20, is still their main transfer target.
