The well-connected Madrid daily published a story at just past 3pm headlined ‘Acuerdo cerrado’ – agreement closed – to relay the news that the two clubs have reached an agreement over the transfer of the Real icon.
Marca also reports that an official announcement was expected late on Wednesday and that Casillas will be treated to a farewell ceremony at the club on Friday.
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Casillas joined Real Madrid as a child and is one of the most decorated players in the club's history, winning five La Liga titles, two Copa del Reys and three Champions Leagues.
The 34-year-old has made 725 appearances for Madrid since 1999 but a notable dip in form - first spotted by Jose Mourinho when he dropped his keeper in the 2012-13 season - has made him expendable at the Bernabeu.
Team-mate Dani Carvajal, who on Wednesday signed a new contract at Real Madrid, backed Spain's most capped international player over his decision to move to Portugal: "I have an exceptional relationship with Casillas, and if he's made that decision, I think it will be the best for him."
News of Casillas' imminent exit will only fuel speculation that Madrid are close to agreeing a deal for Manchester United goalkeeper David de Gea.

Spain's goalkeepers Iker Casillas (L) and David De Gea warm before their international friendly soccer match against Costa Rica at the Reino de Leon stadium in Leon, northern Spain, June 11, 2015. REUTERS/Eloy Alonso

Image credit: Reuters

United have played hardball over the player primed to replace Casillas for club and country, reportedly demanding a world record fee for a goalkeeper in an attempt to extract the most money possible for De Gea, who is out of contract next summer and could then leave on a free.
However, with Madrid ready to sell Casillas, there have been reports in Spain that an agreement is also in place to bring De Gea to the Bernabeu.
A report in El Confidential claimed Madrid will pay £29m to sign the 24-year-old, who was named United's player of the season for 2014-15.
It was once thought that Casillas leaving Madrid would be impossible, such was his iconic status at the club. But a two-year downturn could not be ignored any longer. Florentino Perez loves to collect forwards but Casillas' poor form was hurting Madrid too often and signing a keeper of De Gea's calibre will be just as important as any superstar signing they could make this summer. The goodbyes on Friday will be genuine and emotionally charged, but it is unmistakably time for Casillas to make way for a younger and better option.
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