The Spanish aristocrats have been unable to agree a deal with the 34-year-old Argentina international which will allow them to comply with LaLiga's financial regulations, leading to a parting of the ways.
Here, the PA news agency takes a look at some of the issues behind a momentous decision.
How did it come to this?
We should perhaps not be too surprised. Messi signalled his intention to leave the Nou Camp last summer, believing he could do so on a free transfer under the terms of his existing contract. A 700million Euros – around £629million – buy-out clause prevented him from doing so, but the seeds were sown regardless of the club's financial difficulties.
Are Barcelona really that badly off?
In a word, yes. Spanish newspaper El Mundo reported earlier this year that the club's debt stood at 1.173billion Euros – £850million – and their wage bill currently accounts for 110 per cent of their income, a situation president Joan Laporta knows means they are on the wrong side of financial fair play regulations.
How expensive is Messi?
Very. Reports suggest Messi's basic salary amounts to 45million Euros, or £38.2million, and that he was willing to take a cut of more than 50 per cent to sign a new deal. Barca's statement claims the two parties had reached an agreement, but that the LaLiga rules simply do not allow it to be completed.
Is the club's announcement a bargaining ploy?
A good question. There is little doubt that Messi's profile has promoted LaLiga's product to a worldwide audience and his battle with Cristiano Ronaldo for the title of the greatest of all time is the soap opera which keeps giving. The governing body will not want to be seen as the villain of the piece, the role in which they appear to have been cast by the Nou Camp hierarchy. However, whether that and the public outcry which will inevitably follow the news will prompt a relaxation of the rules remains to be seen.
If not Barcelona, then where next for Messi?
Manchester City and Paris St Germain are two of the few clubs in world football with pockets deep enough to contemplate a move for a man whose advancing years are unlikely to provide a barrier to his future employment. Major League Soccer, where his enduring quality would enable him to play on for many years more, could provide a halfway house towards a return to his roots in Argentina, but the romantics in Rosario will inevitably pin their hopes on Messi's suggestion that he would one day like to return to the club where he launched his glittering career, Newell's Old Boys.