English Heritage sites

Stonehenge, Wiltshire Osbourne House, Isle of Wight Dover Castle, Kent Kenwood House and Art Gallery, London Fountains Abbey, North Yorkshire Battle Abbey, Sussex Bolsover Castle, Derbyshire Beeston Castle, Cheshire Clifford Tower, York Hadrian's Wall, Northumberland

But Katherine Burn, of the union Prospect, said: "It will also affect work on planning applications, and a whole range of other work.

"It's not just front-of-house staff who are involved, it's listed-building planners, architects, surveyors, conservators, people who run the blue plaque scheme, and so on."

A union spokesman told BBC News Online that modest offers were accepted in previous years in the expectation of a better package this year.

Prospect balloted about 500 members at English Heritage.

Nearly 73% responded: 89.6% voted for industrial action, and 76.3% said they would be willing to strike.

Two ballots

Professional staff such as architects, curators and engineers, belong to Prospect, while the Public and Commercial Services Union (PCS) looks after administrative and security staff.

Both have been holding ballots.

The PCS outcome is expected soon.

It is thought to be the first time a strike had been threatened at English Heritage.

English Heritage said a deal gave staff rises of between 3.2% and 4.3% and was all the organisation could afford in a "financially challenging" year.