More than 250 conservators, archivists, administration and other specialist staff will picket five NML sites. Their action follows the highly successful strike on 25 July which launched their pay campaign.

The staff are members of the Prospect and Public and Commercial Services unions. They are fighting an imposed pay offer worth 1.8% for 2007 and 1.3% for 2008, less than half the rate of inflation.

Paul Noon, General Secretary of Prospect, will be joining the picket lines. He said: "The museum’s pathetic pay offer does no justice to our members, the museum itself or the great city of Liverpool.

"Next year Liverpool is the European Capital of Culture and the museum will be promoting the city to millions of visitors. To impose a pay cut on staff for two years running while asking them to promote the city’s image at home and abroad is nothing less than cynical exploitation. We are asking the museum to show some respect for the professional skills of its staff, get back into talks and end the inconvenience to the public."

NML has refused to support a union request for a joint approach to the Treasury for funds to finance a higher cost of living pay offer. It has admitted that it deliberately sets rates of pay "at the lower end of the market" and that NML "is regarded as being a low pay employer."

Bob Wareing, MP for Liverpool West Derby has put down an Early Day Motion in the House of Commons, calling on NML to return to pay negotiations, which has attracted support from 18 members.

The five NML sites that will be picketed on 22 August are: The Walker Art Gallery, World Museum Liverpool, National Conservation Centre, Sudley House, and The Lady Lever Art Gallery (Wirral). Because of Thursday’s opening of the International Slavery Museum, Prospect will not be picketing the Maritime Museum.