Workers at the National Gallery are going on indefinite strike in a long-running dispute over privatisation.

Members of the Public and Commercial Services union at the gallery, in London, have staged a series of walkouts in recent months in protest at visitor services, including security, being privatised. The dispute worsened when a union rep, Candy Udwin, was sacked.

Picket lines were mounted outside the gallery in Trafalgar Square on Tuesday to launch the action.

The Labour leadership contender Jeremy Corbyn sent a message of support: “National Gallery workers have been forced to take this action because of the intransigence of management. These low-paid workers are proud public servants and don’t want their jobs sold off.

“The management of the National Gallery must reinstate Candy Udwin and call off plans to privatise the staff who look after our cultural assets. I urge the incoming director Gabriele Finaldi to meet with the union and resolve this dispute.”

PCS’s general secretary, Mark Serwotka, said: “We had asked the new director to step in to resolve this dispute before taking over but now his first week will be greeted by a continuous strike. We remain ready to negotiate.

“We do not believe this privatisation is in any way necessary and we fear for the reputation the gallery rightly enjoys around the world as one of our country’s greatest cultural assets.”

The gallery has insisted no jobs will be lost.