Porto submit a written offer for Real Madrid's Iker Casillas

Real Madrid goalkeeper Iker Casillas is set to leave the club after 25 years of service in the Spanish capital, TVE have reported. 

The goalkeeper will move to Julen Lopetegui's Porto, where there are already a number of other Spanish players playing under the orders of the former Spain U21 manager. 

Porto have already submitted a written offer to Real Madrid, who are hopeful of signing Manchester United stopper David de Gea this summer. 

TVE's reports suggest Madrid would not stand in Casillas's way and would make up the rest of his salary -- he has two years on his deal at the Bernabeu -- if Porto can't afford to pay it all, as they previously did when Raul moved to Schalke. 

De Gea returned to England on Sunday night to begin pre-season training with United, but the former Atletico Madrid player is keen on forcing through a move to Madrid. 

Casillas, 34, would therefore become second choice at the club, so he may see a move to Porto as the right move at this stage in his career. 

He has previously been linked with Arsenal, Roma, Fenerbahce and even Valencia.