An Algerian chemist was found guilty of contaminating food and wine by spraying a potent mixture of his own urine and faeces in supermarkets.

Sahnoun Daifallah, of White City, also sprayed the substance over children’s books and in a pub as he carried out his campaign by concealing a 1.5 litre weed killer container in a lap top computer bag modified to allow the nozzle to poke out.

Daifallah, 42, was found guilty of four counts of contaminating goods at Tesco, Morrisons, Waterstones bookstore and a pub in Gloucestershire on May 14 and 16 last year.

A jury at Bristol Crown Court believed the prosecution’s case that Daifallah had fantasies about biological weapons.

He was also found guilty of having an offensive weapon, namely a catapult with marbles.

He first visited the Air Balloon pub, near Cheltenham, at 12.45pm on May 14, where police were called after he asked a barmaid how much it would cost to rape her. When officers arrived, Daifallah had gone, but he left a stench which prosecutor Stephen Dent said was his calling card.

He moved on to Waterstones bookstore in Cirencester where he sprayed the brown substance all over a toilet.

Staff noticed the smell but it was not until after he had left the store that they discovered an area of 38 shelves had been doused in the foul substance.

Two days later, at 11am, Daifallah visited the Tesco store in Quedgeley where a shopper saw him reach into his bag and produce a jet of brown fluid over the frozen chips.

He moved on to the wine section, where a member of staff saw a fine vapour come out of his bag and on to the wine, leaving the brown substance over the shelves.

Daifallah then drove four miles to the Morrisons store in Abbeydale, where an employee in the wine section noticed him acting strangely and gagged at the overpowering stench.

Both supermarkets were cordoned off and shoppers were locked in for public safety reasons while the source of the contaminant was traced.

The stores were closed for two days for cleaning, costing about £700,000.

Police officers called by staff at Tesco identified Daifallah on CCTV and arrived at his home in Bibury Road, while he was still in Morrisons.

On searching the flat, they found a several bottles of the noxious mixture and plastic sachets containing excrement.

They also found messages scrawled all over the walls referring to biological weapons, smuggling uranium into Britain and micro-organisms being spread.

A map of Gloucester with “Contaminated 83% Ammonia” written over the city was also found in his bedroom.

During the trial the jury was told that Daifallah had been questioned by police about four incidents in February last year when brown liquid was sprayed in the toilets and around the tables of four pubs in Stroud.

Daifallah, who chose to represent himself in court, claimed that someone else was responsible for the messages and the spraying.

Judge Carol Hagen adjourned sentencing to a date to be fixed to allow the preparation of psychiatric reports.

Daifallah is likely to be deported.

She said: “I find this a very worrying case and the circumstances are alarming.”

She warned Daifallah that if he did not co-operate with the authorities in creating pre-sentence reports, it would “reinforce my conclusion that I find you to be a potentially very dangerous man”.

The White City man might be deported.