Hospital workers are being fined for parking in visitor spaces when the staff section is full.
UNISON said that some employees at Pinderfields Hospital in Wakefield had been penalised for using spaces allocated for patients and their relatives, despite being entitled to a space of their own.
Problems with parking at the site first became apparent last summer, when cars were seen being left along Aberford Road on a daily basis.
It was then revealed that the Pinderfields car park was struggling to accommodate extra numbers of staff who'd been recently employed at the hospital.
UNISON's regional branch secretary Adrian O'Malley said the problem had still not been resolved.
He said: "Some staff have still not been granted car parking spaces. A lot of those people started working at Pinderfields after services moved from Dewsbury.
"Some are on a waiting list but they're having to park on nearby streets and walk. It's a massive issue.
"Even some of those with parking spaces are getting to work and finding the staff car park full, so they have to go into the visitors' section instead. They are getting warnings and being fined.
"If they find that the staff car park's full, what else are they supposed to do? They don't want to be late for work.
"Some are paying around £20 a month for a space and that's a lot of money."
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The Mid Yorkshire NHS Hospitals Trust, which runs the site, was given planning permission to expand the car park by an extra 47 spaces in November.
This followed a separate application for an extra 82 spaces within the hospital grounds, which was approved last April.
In its application, the trust acknowledged that demand for parking spaces is exceeding supply, and said it was endangering road users.
Responding to UNISON's comments, the trust's director of estates and facilities, Mark Braden, said: "Staff car parking permits are car park specific to enable better utilisation of the spaces available.
"If a member of staff chooses to park in a different car park, they may be liable for any charges incurred.
"We recognise parking capacity is an issue for our staff, which is why the Trust has recently obtained planning permission to increase parking capacity on the Pinderfields site.”