The UK's largest retailer Tesco was forced to close 14 stores in locations across the country at the weekend following a major security alert.

The precise reason why the stores, which reopened yesterday (15 July), had to be closed is not yet clear.

A criminal investigation has been launched by Hertfordshire Police - the retail giant has its headquarters in the county - who urged shoppers to be "vigilant at all times".

Amid the current terrorist alert in the UK, there were fears that the Tesco closures were linked to recent terrorist incidents, but a police spokesperson said: "There is still no reason to believe that the incidents are linked to extremism of any kind."

Police are thought to be pursuing extortion as one possible line of inquiry.

Tesco issued a statement apologising for the inconvenience and assuring customers that it had taken advice and was checking additional stores as a precaution, but there was no need to close any other outlets.

"The matter is in the hands of Hertfordshire Police which is conducting a criminal investigation," Tesco said. "Customers and staff can be confident that their safety and well-being remains Tesco's primary concern at all times."

The retailer added that trading in the affected stores on Sunday had been busier than usual, but declined to comment further on the matter.