The raider entered Morrisons in Glevum Way, Gloucester, and fired a "foul-smelling substance" - thought to be urine - on meat, salad and fruit.

He fled the store and went to Tesco four miles away in Quedgeley, where he again terrified customers and staff by spraying the fluid on produce.

A 42-year-old man from Gloucester was arrested at the Tesco store on suspicion of causing criminal damage and remains in police custody. One man is thought to be behind both incidents.

Police do not believe the substance poses a risk to public safety but the shops have been sealed off while Environmental Health officers carry out inspections.

Staff are waiting in canteens of both stores, which are shut, and witness statements have been taken from several customers who saw the discharge of the liquid.

A Gloucestershire police spokesman said: "It is alleged that a man was spraying a foul-smelling substance, thought to be urine, on the foodstuffs, including the salad bar, fruits and meat.

"The device he was using is described as a kind of tube. Clearly this is a very unusual incident. A man has been arrested and a full investigation is under way. We are appealing for any witnesses who have not already contacted us to come forward.

"The substance is believed to be of no threat to public safety."

Officers say they have no clues about the motive.