A dog owner is facing prosecution after his bullmastiff mauled another dog on Hampstead Heath.

The out-of-control animal savaged a shih tzu near the Kenwood Ladies’ Bathing Pond at about 1pm on Monday.

Alarmed dog walkers took to social media to warn others to be on their guard after hearing of the attack.

The shih tzu is understood to be seriously injured and facing an operation, although rumours that it was killed in the attack were untrue.

The Hampstead Heath Constabulary plans to prosecute the owner under byelaw 21, which says dogs must be kept under control and restrained from injuring people, animals or plant life.