Dozens people were taken to hospital after a bizarre gas attack today on a chain of DIY stores in St Petersburg, Russia’s second city.

A spokesman for the Emergency Situations Ministry said that 78 people were said to have been “sickened” by the release of the gas in a shop belonging to the Maksidom chain. Sixty-six of them were hospitalised, but most were soon released.

Employees in three other Maksidom stores found boxes with wires and timers attached to containers, but the devices were safely neutralised. The attack happened at the height of the pre-New Year shopping season.

"There were devices made out of ampoules containing a so-far unknown substance, which on dispersal gave out a strong pungent smell like garlic," said a spokeswoman for the local Federal Security Service, the successor of the Soviet-era KGB.

Jeremy Page, Moscow Correspondent of The Times, said that the gas involved had been identified by local media as methyl mercaptan, a foul-smelling compound mostly commonly used to give domestic gas its odour.

Page said: “This is the same substance that skunks secrete, that is found in faeces and is used by kids in stink bombs. It can make you gag or be sick, but it’s relatively harmless in small concentrations.

“The FSB is describing this as a criminal attack rather than some kind of terrorist attack. It looks like an attempt to discredit this particular chain of shops.”

A St Petersburg police spokesman told the Interfax news agency: “The likely explanation for what happened is that it was malicious competition.

“Not long before the incident, the managers of Maksidom appealed to the police because they had begun to get letters with threats to disrupt pre-New Year trade. Now we can definitely say that the possibility of a terrorist act is ruled out."