Hundreds of staff at London's National Gallery on Tuesday began an indefinite strike, the latest in a series of walkouts to protest at the outsourcing of some services to the private sector.
Members of the Public and Commercial Services (PCS) union have staged 56 days of action since February after plans were revealed for privatising certain services, which the gallery said would allow it to "operate more flexibly and deliver an enhanced service".
The museum is London's second most popular tourist attraction, drawing around six million visitors per year.
It will stay open during the latest strike, although some rooms will be closed and many educational workshops have been cancelled.
Gabriele Finaldi, the museum's new director, announced in late July that he had signed a five-year contract with the private company Securitas to look after reception and security, covering around 300 current positions.
The museum insists that no jobs are threatened, but PCS leader Mark Serwotka argued that privatisation could "harm the (gallery's) international reputation."
Around a third of the museum's 600 staff are expected to join the protest.
Nick McCarthy, the union's director of campaigns and communications, told BBC radio: "We have no alternative but to go on strike -- the privatisation is completely unnecessary.
"Today's strike is indefinite until such time as we are able to reach a solution with the gallery."
There will also be a picket line outside the gallery.
The union has received the backing of Jeremy Corbyn, the leftist candidate who is odds-on favourite to become the next leader of the Labour Party.
He denounced the "intransigence" of museum management and called on them to drop the plans.
The gallery's imposing building overlooks Trafalgar Square in the heart of London and its impressive permanent collection of paintings from the 13th to 19th centuries is open to view without charge for 361 days a year.