Porto have submitted a formal offer for Iker Casillas, according to Spanish television channel TVE.
The Portuguese club have set their sights on the Madrid No.1 to be their regular starter next season and Madrid are currently mulling over an offer for the 34-year-old.
Casillas has previously insisted he wants to stay at Madrid, even if he plays second fiddle to David de Gea, but a move away from the Spanish capital now seems likely.
Porto could be seen as the ideal move with Madrid already having good relations with the club and Porto having Champions League football next season.
Arsenal had previously been linked with the Spain international but their addition of Petr Cech from Chelsea ended their pursuit.