Used syringes being dumped in postboxes around Sheffield

This postbox near Sheffield Cathedral is among those affected

The slot has been narrowed from a few centimetres to a matter of millimetres at selected locations to protect postal workers from stabbing themselves on needles when collecting mail.

Royal Mail said the change had been made to a 'small number' of postboxes around the city, though it did not say exactly how many had been adapted or where these were.

This postbox at Heeley Green is another which has been adapted to prevent needles being dropped through the opening

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Royal Mail has apologised for any inconvenience the change may cause customers but said it had to protect its staff from injury.

Royal Mail spokesman Henry Perry said: "We can confirm that a small number of postboxes in Sheffield are currently fitted with restrictor plates to stop items other than letters being posted into them.

A council poster advising people how to dispose of used needles properly

"We apologise for any inconvenience this may cause our customers. Any customers who have larger items to post can do so at any Post Office branch or at alternative postboxes."

Royal Mail did not say how often needles had been found in postboxes or why the locations had been chosen, though the area around Sheffield Cathedral is a well-known hot spot for drug abuse.

Needle exchanges are available at several pharmacies around the city and at other locations including the Cathedral Archer Project on Campo Lane, which runs a drop-off only service.

Posters are displayed around the city help users find their nearest needle exchange, where they can collect small bins for personal use and drop them off once full.