
The Hoops have been told they will have to move quickly to secure the services of the promising Real Sociedad left-back.
Castillo is out of contract at the end of this month and, whilst a Spanish side would be forced to pay £2million for him, a foreign club can sign him for £100,000.
The 23-year-old said: "I have interest from Italy, England and Greece, but what I want is a proposal from Celtic."
However his agent, Joaquin Barcena, last night insisted the Hoops will have to move quickly with French giants Marseille the latest side to express interest.
He said: "Xabi has instructed me to find a club with a 'British' accent and already stated Celtic would be perfect. Others have enquired about him, though, so nothing is certain yet. Hopefully, we can come to an agreement soon."
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