Celtic will pay an initial £415,000 for the 20-year-old, who made his first-team debut in 2006, rising to a potential £1.6million depending on appearances.

The deal, which is expected to be finalised in the coming days, is the culmination of talks between the clubs which were first initiated four years ago by Celtic chief executive Peter Lawwell and then manager Martin O'Neill.

The complex transfer gives Barcelona the right to buy back Crosas in two years for £1.65million with the Glasgow club retaining 25% of his worth.

If the Catalan club do not take up that option, they would receive 25% of any transfer fee Celtic receive from another club.

Crosas, who helped Lyon win the French league and cup double in a loan spell last season, is the first player to move from Barca under the terms.

But Lawwell has revealed that Argentina forward Javier Saviola and Portugal winger Ricardo Quaresma earlier refused the opportunity.

"We would have taken Quaresma and Saviola but the players decided not to come," he told the Sunday Herald.

"We have always thought that this formula could be successful. So we have looked at Crosas.

"We are not finally there but that deal is now a structure which will hopefully allow us to add players going forward.

"Hopefully it makes a lot of sense. Not only with Barcelona but with other clubs too.

"A young or fringe Barcelona player could come here, play in our first team, play in front of 60,000 and in Europe and develop as a player. Both clubs could benefit from that."