Emergency services responded to reports workers had been attacked at an Asda in Barking (Credit: Google)
A member of the public reportedly broke into the supermarket, in Short Blue Place, while it was closed and assaulted two members of staff at around 11.59pm on Saturday, 18 June.
Emergency services took the victim to hospital with a head injury. He is still in a critical condition.
The second victim was not seriously injured.
Police arrested a 27-year-old man on suspicion of GBH at the scene. He remains in their custody.
Detectives are continuing to investigate the incident.
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Tags: Barking, Barking and Dagenham, Assault, Metropolitan Police, Attack