Landmarks such as Hadrian's Wall are among tourist attractions that could be be hit by strikes over job losses and pay during next week's summer solstice.
Hadrian's Wall, built by the Romans, is a World Heritage Site
Almost 500 English Heritage (EH) workers will walk out on 21 June, their trade union, Prospect, has announced.
Union official Dave Allen described an imposed pay rise of 1.5% as "poor".
EH insisted no sites would close as a result. Stonehenge will have a scheduled closure on 21 June following annual all-night solstice celebrations.
Workers also plan to walk out from English Heritage's head office in London and several regional offices, including Swindon, York, Guildford, Cambridge, Newcastle, Manchester and Bristol.
'Make a stand'
Workers based in northern England will leave their posts for two hours in the morning, while their southern colleagues will stop work between 1400 and 1600.
Mr Allen said proposed job cuts "strike at the very heart of the organisation's ability to extend its educational remit into schools and colleges".
He said staff had been forced to make a stand.
An EH spokeswoman said Stonehenge would be totally unaffected because the site closed every year following the summer solstice gathering.
She said they expected around 21,000 people for the celebrations, starting at 2200 BST on 20 June and finishing at sunrise on 21 June.
The site is cleared and tidied up from 0900 and reopens on 22 June, she added.