Between 6pm and 6.30pm on July 27, a suspected shoplifter ran out the Co-Op in Twyford Avenue with goods, chased by a security guard.
The security guard tried to detain the suspect who lunged at him with a knife.
The guard managed to block the suspect, who then threatened him with a syringe.
The suspect dropped goods, worth £75, and ran away.
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The guard was not hurt in the incident.
Anyone who knows the man in the CCTV is asked to call 101 quoting 44170289401.
The Southern Co-operative has been approached for comment.