Workers at the National Gallery will stage another strike today in a continuing row over jobs.
Members of the Public and Commercial Services union will walk out for two hours from 1pm following a number of stoppages in recent weeks.
The workers claim that staff cuts mean they are no longer able to do their jobs properly, adding that the Government axed 15% from the gallery's budget as part of its spending review in October 2010, amounting to £4 million for the period up to 2014/15, £1.5 million through staffing.
The union claimed the gallery, in London's Trafalgar Square, was doing all it could to keep open a Da Vinci exhibition, for which there was a charge to enter.
PCS general secretary Mark Serwotka said: "Instead of doing everything they can to maintain fee-paying exhibitions at the expense of the free and publicly available collection, gallery managers should be talking to us about ending this dispute and restoring staffing to acceptable and safe levels."
A gallery spokesman said: "The National Gallery has increased the number of rooms in which a gallery assistant invigilates to two rooms rather than one. This is not unusual or controversial in the museum and gallery field.
"Quite the contrary - the majority of galleries within London, throughout the UK, across Europe and far beyond all employ similar systems.
"The change has been made because the National Gallery and others believe this to be a more effective and reliable means of invigilation and it has been implemented with the full approval of the current national security adviser.
"The security of the collection is of paramount concern to the National Gallery."