Workers at the National Gallery in London are staging a fresh wave of strikes in a long-running dispute over privatisation.

Members of the Public and Commercial Services union will take industrial action this week and again on 1 May, having already walked out for 17 days in protest at plans to sell off visitor services.

The union said an invitation to tender is due to be published days before the general election, and before the new director, Dr Gabriele Finaldi, takes up the post in August.

The PCS general secretary, Mark Serwotka, said: “It is cynical of the gallery to try to press ahead with this sell-off in the runup to the general election and before the new director has taken over. The process should be halted now and we should be given a proper chance to discuss maintaining services in-house.”

A National Gallery spokesman said: “The National Gallery is a public asset and has a duty to ensure the collection, and the gallery itself is accessible as much as possible to as many people as possible.

“The PCS opposes the introduction of a new roster for some visitor-facing and security staff which would enable us to operate more flexibly. In conjunction with the new roster we also proposed not only to meet the London living wage, but to pay a basic salary in excess of it. As a result of the PCS position, we are now appointing an external partner to manage these services. Affected staff will transfer across; there will be no job cuts and terms and conditions will be protected.

“We are one of the last major national UK museums to take this step. We believe the proposed changes are essential to enable us to deliver an enhanced service to our 6 million annual visitors for many years to come, and to remain as one of the world’s leading art galleries. It is unfortunate the PCS union do not share this aspiration with us.”