Postboxes in Sheffield have had to be adapted to prevent drug addicts dropping used syringes through the opening.
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.
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The postboxes affected include one near Sheffield Cathedral and another outside Heeley Post Office, where it is now only possible to fit the slimmest of mail through the gap.
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.
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"This is in response to needles being placed in the postboxes, which risk injuring our collections postmen and women, and can also damage customers' mail. We are monitoring the situation and will review in due course.
"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.
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Sheffield Council advises drug users to dispose of their syringes safely and responsibly, as dumping them in public places puts other people at risk.
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.