Coffee drinkers must clean reusable travel cups after every use to avoid potentially harmful bacteria building up inside them, Public Health England has said.

The warning appears to be directed at a rising number of office workers who are thought to be using the cups continuously throughout the day without washing them.

Reusable coffee cups' popularity has exploded in recent weeks, amid Government plans for a 25p "latte" tax on disposable cups.

A number of major coffee chains have also now started offering special discounts on drinks to incentivise the use of reusable cups, causing sales of the drinking vessels to rocket at major retailers.

Nick Phin, deputy director for national infection service, at PHE, said: “As with regular cups and glasses, wash and clean reusable cups thoroughly after every use.”