"The deal will be signed in the coming weeks, when Messi returns to the team for pre-season training," said the club in a statement.
The club gave no details about the contract that will keep the Argentinian megastar and five-time Ballon d'Or winner at Camp Nou until he reaches the age of 34.
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But Spain's Marca sports daily reported on Tuesday that it included a €300m termination clause.
His current contract was due to run out in June 2018.
The 30-year-old Argentina international joined FC Barcelona in 2000 when he was just 13 years old in the youth training centre.
"He made his first team debut at just 16 years of age in a friendly against FC Porto, before making his debut the following season against Espanyol at 17," it said. "Shortly after, he scored his first senior goal with a clever lob against Albacete at the Camp Nou."
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Now, with 507 goals scored in 583 official matches, Messi is the club's all-time leading scorer and is widely considered one of the world's best players.
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