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

"Previous modest pay settlements have been agreed by the unions on the basis that there would be 'jam tomorrow', at least in terms of a fairer pay system," said negotiating officer Steve Jary.

"But despite previous reviews and last year's commitments, no concrete progress has been made."

English Heritage 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 unions are balloting their members.

Mr Jary said Prospect expects to have the result of the ballot early next week, with the PCS result to follow shortly after.

Stonehenge is a site employing union staff

A considerable number of English Heritage's major attractions would be closed as a result of any stoppages.

Some monuments, including parts of Hadrian's Wall, would remain open to the public.

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.

A spokesman for English Heritage said: "The award is significantly above inflation and comparable with settlements in the wider public sector.

"We very much hope that staff will choose not to take industrial action of any kind."

Mr Jary said he was unaware of any past strikes by English Heritage.

"I don't think we've even come close before. This is a last resort."