The stores were closed on Saturday afternoon after Tesco received 'unspecific' threats. Police searched all the stores and gave them the all-clear. They reopened on Sunday.
The stores affected included those in Bury St Edmunds, Suffolk, Pontefract in West Yorkshire, Port Talbot in South Wales, Market Harborough and Ashby-de-la-Zouch in Leicestershire, Barrhead in Glasgow and Dalgety in Fife. Others were in Ledbury and Hereford in Herefordshire, as well as Hucknall in Nottinghamshire and Barnes in Southwest London.
The police do not think that political, religious or animal-rights extremists are behind the threats. A police spokeswoman said the communication had been 'unspecific'. Links to the Al-Qaeda attacks in London and Glasgow have proved unfounded.
Detectives are also investigating the possibility that the attacks were made by an individual or group with a grudge against the retailer. It is unclear whether money had been demanded from Tesco.
A Tesco spokeswoman said: 'The police are conducting a criminal investigation and Tesco cannot comment further.'
A police spokeswoman said: 'Each of the stores has been searched and given the all-clear and the company continues to work closely with the constabulary. The public are asked to remain vigilant at all times. There is no reason to believe that the incidents are linked to extremism of any kind.'
The incident coincided with a national day of action against the retailer by animal-rights protestors including Vegetarians International Voice for Animals (VIVA), which objects to the retailer selling live turtles in China. The turtles are killed at the counter in Chinese stores or taken home to kill. The retailer said the butchering was carried out humanely.
Separately, factor workers in Bangladesh making clothes for Tesco, George at Asda and Primark have reported alleged physical and verbal abuse by supervisors.
Asda has launched an investigation into allegations that the workers are forced to work for up to 80 hours a week for as little as 4p an hour.