The National Trust has removed a Union Jack from the Penshaw Monument, saying it had been placed there without permission.
The flag appeared on the Grade I-listed landmark overnight on Friday, attached high up on the 70ft structure.
The Trust, which has owned the site since 1939, said it would be carrying out checks to ensure no damage had been caused.
Penshaw Monument, built between 1833 and 1845 in memory of the 1st Earl of Durham, is a replica of a Greek temple and remains one of the most recognisable landmarks in the North East.
The charity stressed that anything fixed to historic sites would be taken down.
