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Messi has spent his entire career at the Catalan outfit, scoring a staggering 672 goals in 778 appearances since making his senior debut in 2003.
But Barca must now brace itself for his exit, with Paris Saint-Germain the front-runner to claim his sought-after and coveted signature.
To put it simply, Messi will leave Barcelona after they were unable to fulfil a new contract due to "financial and structural obstacles".
Barcelona announced that Messi, arguably the greatest player of all time, was set to leave as a new contract could not happen in line with financial fair play rules set by La Liga.
The six-time Ballon d'Or winner has officially been a free agent since July 1, having not agreed terms over a renewal before his previous contract expired on June 30.
The 34-year-old had been expected to sign a new five-year contract with the club, which would have seen his salary reduced by 50 per cent from £100m to around £50m per year - but league regulations made it an impossibility.
"Despite FC Barcelona and Lionel Messi having reached an agreement and the clear intention of both parties to sign a new contract today, this cannot happen because of Spanish LaLiga regulations on player registration," Barcelona's statement read.
"As a result of this situation, Messi shall not be staying on at FC Barcelona. Both parties deeply regret that the wishes of the player and the club will ultimately not be fulfilled."
During an emotional farewell news conference on Sunday afternoon, Messi broke down in tears before announcing his departure.
"This is the toughest moment of my career. When the club told me, I just froze. This is all like a bucket of cold water has been poured over me and we are still coming to terms with it," he sobbed.
Quickly emerging as a front-runner to sign Messi, PSG have tabled a lucrative two-year deal, thought to be worth £25m-a-year after tax - with the 34-year-old now considering a move to the French capital.
When asked if a move to PSG was on his mind, Messi said: "That is one possibility, to reach those heights."