Britain's senior counter-terrorism police official has warned that some social media companies undercut efforts to thwart plots by tipping off suspects about police inquiries.

Mark Rowley said Monday that some suspects have been told when police request their communications data.

He said some companies refuse to provide data to police when legitimate requests are made.

Rowley asserted that as a result there are more "blind spots" than ever before, making it more difficult for police to track potential extremists.

He told the Royal United Services Institute that some social media companies are cooperative but others have intentionally designed their products so that they will not be able to provide information to authorities.

Rowley did not name any companies that routinely tell users that police have requested their data.

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