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Members of English Heritage are no longer able to use Deal Castle as an all-day town centre car park.
They were previously allowed to leave their vehicles in the spaces at the castle all day but will now have a three-hour limit.
English Heritage says the move follows complaints of the site being used as a long-term car park for Deal town centre, with some vehicles being left there on a permanent basis.
A notice says the new rule will come into force from April 1 'at the latest'.
English Heritage owns the land and Dover District Council runs the car park.
The charity agreed to the change to keep spaces free for visitors to the castle.
One English Heritage member, Neil Oldfield of Walmer, said: "Why is the council seeking to limit the amount of time I can park in Deal Castle car park?"
He says he only discovered the change by coming across a notice further away, in Canada Road, Walmer.
The notice, dated February 26, has the announcement of the change among details of other proposals concerning parking areas in the Dover district.
The others include a list of places for introducing electric vehicle charging points in Dover, Deal and Sandwich and starting residents-only permits in two Dover streets.
A spokesman for Dover District Council said: "This change is being introduced at the request of English Heritage.
"We can confirm that a notice has also been placed in the Deal Castle car park."
Mr Oldfield argues that the notice is at the side of the car parks's pay and display machine, which English Heritage members would not be drawn to as their parking remains free.
A spokesman for English Heritage said: "Members receive the benefit of free parking at our sites, for the purpose of visiting the site in which they park.
"Following many instances of Deal Castle car park being used as a long-term car park – and in some cases vehicles being left on a permanent basis, we have worked with Dover District Council to implement a three hour time limit.
"Deal Castle is one of our smaller sites, which takes our members and visitors around one to two hours to explore so the new limit leaves plenty of time for those enjoying the castle.
"When the car park is used for other purposes it leaves less spaces for those planning to visit the castle. As a charity, this means we lose vital income from our visitors, which helps us care for our historic sites – such as Deal Castle.
"There are many other car parks in Deal, plus on-street parking which we recommend locals use when visiting the town."
Non-members of English Heritage continue to be charged through the pay and display system and are already limited to three hours.