Tesco has been raided by the UK Border Agency which has discovered the grocer has been employing people for longer hours than their visa allowed.

The grocer is facing large fines after immigration officials raided Tesco’s online fulfilment centre in Croydon, south London last month.

Around 20 foreign students of 11 different nationalities were found to have been working for significantly longer than their visas allowed. Seven of the students have already been deported, The Daily Telegraph reported.

Tesco said it is “co-operating fully” with the UK Border Agency (UKBA) and said that it had tightened procedures to tackle illegal workers, which it did not condone employing.

The students, who were largely university students from Bangladesh or India, had been working up to three-and-a-half times longer than their visas allowed.

The Home Office said that inquiries were “ongoing”. It is understood that a further 15 students remain under investigation.

Tesco could now face a fine of up to £10,000 per illegal worker. UKBA raided the Factory Lane warehouse at 3am on July 21.

A Tesco spokesman said: “In cooperation 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 cooperate 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.

He added: “We have a comprehensive system for ensuring all the correct procedures are followed in this area which has been externally audited and generally works well. We have now taken additional steps to ensure an incident of this nature does not happen again.”