Heritage workers planned to gatecrash the summer solstice celebrations at Stonehenge as part of nationwide strike action over pay.
Revellers gather at the monument for the solstice each year
More than 800 English Heritage staff at sites also including Hadrian's Wall will walk out over a "derisory" 1.5% pay rise, their unions said.
Some were expected to turn out for sunrise at Stonehenge to demonstrate.
But employers said the solstice party, expected to attract 21,000 revellers, would be unaffected by the strike.
Stonehenge is due to be shut all day for cleaning.
No other sites are expected to close but opening hours may vary at certain sites, English Heritage said.
'Enough is enough'
Around 340 Public and Commercial Services (PCS) union members across England - mostly frontline staff at museums and visitor's centres - will stage a two-hour walkout.
They will be joined by 500 workers from the Prospect union, whose members include archaeologists, architects and surveyors.
As well as the Stonehenge demonstration, a further protest is expected outside English Heritage's headquarters in London.
The unions are angry at the 1.5% pay rise, which they say is just 50p a day to some of their staff.
PCS general secretary Mark Serwotka said EH staff were telling their bosses "enough is enough".
"Staff have become frustrated and angry as funding is squeezed and management focus their attention on chasing income from private events and shops rather than on the maintenance of historic buildings and the concerns of staff," he said.
The walkout will also affect regional offices in Swindon, York, Guildford, Cambridge, Newcastle, Manchester and Bristol.
Workers based in northern England will leave their posts for two hours from 1000 BST in the morning, while their southern colleagues will stop work between 1400 and 1600 BST.
An spokeswoman for English Heritage said Stonehenge would be totally unaffected because the site closed every year following the summer solstice gathering.
"The solstice ends at nine in the morning on Tuesday 21 June and Stonehenge will not be re-opening until Wednesday morning so that the site can be cleared."
The Stone Circle on Salisbury Plain, Wiltshire, has drawn visitors for the summer solstice for 5,000 years.
A spokesman for the Druid Network said a strong turnout was expected on Tuesday morning because of the predicted good weather.
"It attracts all sorts," the spokesman said.
"I've seen little old ladies with deck chairs and flasks of tea, party goers, revellers, men dressed in suits of armour and architects."
The sun will rise at 0458 BST.
The Stonehenge solstice has become a traditional stopover for music fans heading for Glastonbury Festival, which starts on Friday.