Manchester City believe they have a chance of signing Barça's Lionel Messi

Manchester City believe they have a chance of signing Lionel Messi after the forward told Barcelona he wants to leave.

Related: Where next for Lionel Messi after his decision to leave Barcelona?

Pep Guardiola was Messi’s manager at Barcelona for four hugely successful seasons from 2008-12 and would love to be reunited with the 33-year-old. He had never anticipated Messi leaving the Camp Nou but City, whose chief executive Ferran Soriano and director of football Txiki Begiristain also worked at Barcelona, will try to secure a deal.

It is understood City are banking on being able to get Messi on a free or for a relatively minimal fee, and a legal battle may determine that. Messi’s camp believes a clause in his contract means he can walk away for nothing. Barcelona, by contrast, insist that he is still bound by a €700m (£629m) buy-out clause.

A clause in Messi’s contract allows him to leave for no fee provided he communicates that decision, which he did by burofax – in effect a recorded letter – on Tuesday afternoon.

Barcelona believe the deadline for that clause to be applied has expired, arguing he had to inform them before the end of May. But given the exceptional nature of this season, which extended into the summer and did not formally end until the Champions League final last Sunday, Messi’s camp are set to argue the deadline should be set on 31 August.

Related: Lionel Messi tells Barcelona he wants to leave but faces legal battle over clause

The club replied by burofax to Messi to tell him they want him to stay. If Barcelona accept that he can leave but not on a free, they will realistically be forced to drop their demand for €700m and negotiate a price.

Sporting director Ramón Planes insisted on Wednesday that Barcelona want to build a team around Messi.

Speaking at the official unveiling of £27.8m (€31m) signing Francisco Trincão, Planes said: “We are not contemplating any kind of departure because what we want is for Messi to stay. We only have enormous respect for Leo Messi. Barça and Leo are like a marriage, where both have given so much to each other and have brought so much joy to the fans.

“As we’ve said many times, as [new head coach Ronald] Koeman and the president have said and my position as technical secretary, we count on Messi for the future, just as we count on Trincão as a talent of the future, and we have heard that he will play alongside the greatest player in the history, the greatest player in the world.

“These things happen and we hope we will come back stronger from the defeat at the end of last season. What’s happened has happened and our idea is to build a team around the most important player in the world.”

On Tuesday night several hundred Barcelona fans gathered outside the Camp Nou chanting for Messi to stay and for the resignation of club president, Josep Maria Bartomeu, who has no intention at the moment of resigning.

City will not be alone in trying to sign Messi, whose salary is understood to be about £500,000 a week and who has one year left on his contract. Paris Saint-Germain are expected to be among other interested clubs.