A Tesco store in Barnes, south-west London, was one of 14 branches countrywide that were forced to close after bomb threats yesterday.

It is understood that letters were sent to the stores claiming that bombs had been planted, although police said that they had "no reason to believe that the incidents are linked to extremism of any kind".

Speculation is also mounting that the threats are linked to animal liberation groups who designated yesterday as a national day of action against Tesco.

It is likely to be coincidence, but 14 stores were targeted on July 14.

Thousands of customers deserted their trolleys in the aisles of the largest supermarket chain in the UK after some stores told their customers they had to close because of an "electrical fault".

All stores were closed at around lunchtime yesterday afternoon as a "precautionary measure".

It was only later that it emerged a bomb threat had been made.

The Barnes store was the first to reopen last night.

Other branches were closed in Hereford and Ledbury, both in Herefordshire; Hucknall in Nottinghamshire and Bury St Edmunds in Suffolk.

Further north saw stores shut in Pontefract, West Yorkshire; Market Harborough and Ashby-de-la-Zouch in Leicestershire; and Barrhead, Kirkcaldy and Dalgety Bay in Scotland.

In south Wales, closures included stores in Dyfed Powys and Port Talbot.

Police said other stores were closed in regions covered by their forces in Lancashire and Humberside.

Hertfordshire Police is leading the investigation because that is where Tesco has its headquarters.

In a statement last night, it said it had "launched a criminal investigation following a series of threats made to 14 Tesco stores across the country that occurred today".

"Police were alerted immediately and have liaised with Tesco throughout the day. Public safety has been of paramount importance to all involved."

The statement also confirmed there had been no injuries related to the bomb threats and hoped the stores will be reopened by tomorrow.

Hertfordshire police also stated: "While the public are advised to remain vigilant, we have no reason to believe that the incidents are linked to extremism of any kind.

"The constabulary has taken the investigation forward as Tesco has its head office in Hertfordshire. Every effort is being made to trace those involved."

A worker at the Tesco Metro store in Dalgety Bay, Scotland, said: "I was working on the check-out, which was really busy, when we were told over the tannoy there had been an electrical fault and to go to the fire assembly points.

"All the customers were told to leave the building immediately, and literally had to leave their trolleys and baskets where they stood. The store was shut and we were told to go home.

"It was later on that we spoke to senior staff and we were told there had been some kind of bomb scare. The rumour just spread around the staff although some of us thought it might be a prank."

William Lau, manager of a Chinese restaurant next door to the Tesco store, which is near the police station, said: "The police were here twice and told me a suspicious package had been handed in. They didn't say how or where it had been handed in, and didn't give me any more details. I think they did a search of the supermarket but didn't find anything.

"Everybody is a bit tense after the Glasgow airport attacks. We don't need this to happen."

A meeting took place in Hertfordshire last night where police, Tesco and law enforcement agencies discussed how they would co-ordinate the investigation.

The inquiry will include gathering forensic evidence and recovering the letters in question which have been received, so far, by 14 stores.

A Tesco spokeswoman said: "Tesco is working with the police following a series of incidents. Police are investigating and so we are unable to comment further."

She added: "As always, at all times customer welfare is our priority."

There has been a mounting public backlash against Tesco as it now controls 30% of the grocery market in the UK, with 1988 stores and 40,000 workers.

The chain announced profits of £2.5 billion for the past year.

Groups such as the Tescopology Alliance campaign against the control the supermarket has and its dominance over smaller, local shops.

They claim the Tesco's continual expansion "poses serious consequences" for local farmers, employers, local producers and retailers as well as the environment.

They have called for a block to be put on Tesco takeovers such as its move to buy gardening retailer Dobbies.