Tesco, one of the country’s big four supermarket chains, faces fines of up to £200,000 after UK Border Agency officers found its staff were breaching the terms of their work visas.

An investigation by the UKBA found that foreign students were working longer hours than their visas allowed at the firm’s Croydon warehouse, which supplies groceries for online customers.

Agency officers arrested 20 employees of mostly Indian and Bangladeshi nationality and it has been reported that seven of them have been deported.

This group of foreign students had the right to work in the UK but only for 20 hours a week. However, officers found the students had been working between 50 and 70 hours, supposedly in tandem with studying an educational course.

Tesco has been issued with a ‘notice of potential liability’ by the agency and fines could be as much as £10,000 per worker.

In a statement Tesco said: “In co-operation with Tesco, the UK Border Agency visited our dot com store in Croydon in July.

“As a result of this visit, a small number of staff were found to have breached the terms of their working visas. We continue to co-operate fully with the UK Border Agency as they look into this issue.

“We take our responsibilities as an employer very seriously and do not condone illegal working of any kind.”

The employer said it has “a comprehensive system” for ensuring all the correct procedures are followed in this area which is externally audited.

The statement also said Tesco has taken “additional steps” to prevent a further incident of this kind.

A spokesman for UKBA said: “We received information that some staff members were working in the UK illegally at Tesco.com on Factory Lane, Croydon.

“In response officers carried out an operation in full cooperation with the company shortly after 3am on Saturday 21 July 2012. Twenty individuals have been arrested and now face removal from the UK.”

“The operation was part of an ongoing campaign to tackle visa abuse which has seen over 2,000 offenders removed since the beginning of May.

"The employer now needs to provide evidence that it was carrying out the legally required checks to avoid a fine.”