Middlesbrough have complained to the police over racist abuse directed towards striker Chuba Akpom on social media.
The Championship club contacted Twitter over the content of the post but, in a reply, the platform deemed it not to be in violation of their rules.
Boro have called Twitter's position "completely unacceptable".
As a result they have reported the matter to Cleveland Police to investigate.
A statement from the force said: "Cleveland Police is investigating a report from Middlesbrough Football Club of a racist remark made towards a man on social media.
"Enquiries are ongoing.