The staff, represented by the Prospect union, voted overwhelmingly by 83% in favour of strike action and 91% in favour of action short of a strike.

The result will see members walk out at NMSI’s sites – the Science Museum, London, the National Railway Museum, York, the National Media Museum, Bradford, and the Science Museum, Swindon.

Despite lengthy negotiations with NMSI management and agreements to stay previous planned industrial action, Prospect says its members are furious and their patience has been exhausted.

National Secretary Emily Boase said: “The deal they have imposed is the worst offer of any heritage site in the UK, falling even below the spending levels set by the Treasury. We see this as insulting as there is no excuse for the museum’s failure to offer its staff a fair deal.

“Members have endured a pay freeze after waiting more than a year for an offer to be tabled. They now face an effective cut in pay with the imposition of an offer of just 1.5% plus performance related additions in the first year, with the second year being based entirely on performance levels.”

Prospect members suspended industrial action planned for February this year in the hope that negotiations would secure a decent pay offer for 2007 – outstanding since April 2007 – and 2008. Further negotiations sought to seek a resolution to the dispute after members rejected the offer tabled in April.

The union argues that NMSI failed to budget for an adequate deal, having only allowed for a rise of 3% for 2007 and 2.7% in 2008, of which only 1.5% was set aside for a cost of living increase in 2007 with nothing allocated for 2008.

NMSI warders, security and administrative staff represented by the PCS union have also voted in favour of strike action.