Manchester United hold an option to extend Rio Ferdinand's contract which was thought to expire at the end of the current campaign.
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The clause, which is understood to be similar to the one which the club had with Dimitar Berbatov, means that the former England international is not necessarily a free agent at the end of the season as suggested.
West Ham have already been linked with a summer move for Ferdinand, with manager Sam Allardyce admitting he would be interested if the defender left Old Trafford. And there would likely be lucrative interest from abroad in the 33-year-old's signature.
Sir Alex Ferguson has already hinted that Ferdinand could be set to extend his stay with United, despite the duo's public falling out recently over the player's refusal to wear a Kick It out t-shirt.
"I think Rio can play for two or three more years," Ferguson said recently. "He is not as quick as he was but that is not a big issue for me. His experience is important and there is no reason he can't stay on. I didn't even realise his contract was up."
An ESPN source said: "Rio has this year and one more year on his present contract. It is a kind of an option."