The warning was issued after a Facebook user showed clothes on makeshift laundry lines in the premises of Wat Khok Phraya, a national heritage since 1980.
The name of the Facebook account and the time the pictures were taken were not disclosed.
Sukany Baonid, the director of the Phra Na Khon Ayutthaya Historic Park office, said she had talked to residents living near the site in Phra Nakhon Sri Ayutthaya district and they admitted that it was against law.
Officials will give warnings to them and other people who did the same at other national sites, informing them of possible legal consequences.
People who ignore the warnings and continue to dry clothes could be considered violators of the Act on Ancient Monuments, Antiques and Objects of Arts and National Museum, she said.
The law carries the maximum fine up to B1 million and jail term of 10 years or both, the official added.
Wat Khok Phraya was used as a site to execute kings and royal families since King Ramesuan reigning Ayutthaya from 1339-95.
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