Workers at the National Gallery are staging a series of fresh strikes in a row over the privatisation of services.
Members of the Public and Commercial Services (PCS) union at the National Gallery are walking out for five days, following a strike on 2 February.
About 200 workers are campaigning against plans to switch visitor services to a private company.
A National Gallery spokesman said there would be no job cuts as a result of the transfer.
A 40,000-name petition was handed to the Department for Culture, Media and Sport by the PCS supporting its case.
The dispute worsened when union rep Candy Udwin, who was involved in talks at the conciliation service Acas, was suspended on the eve of the strike.
PCS general secretary Mark Serwotka said: "With this privatisation plan, the National Gallery is putting its well-earned worldwide reputation at risk.
"Its decision to suspend one of our senior reps is a disproportionate and unfathomable act of bad faith and not only should she be reinstated immediately, gallery officials must commit to reopening full and proper negotiations."
A National Gallery spokesman said all planned education events had been cancelled or rescheduled due to the action.
He added: "The negative impact this strike action makes on our education events is significant and damaging.
"The most acutely affected area is the National Gallery's programme for schools and teachers, which included daily sessions for primary aged children including children with special educational needs."
The spokesman said: "From April, we will pay staff a top-up which will bring salaries in line with the London Living Wage.
"As a result of the PCS position, we will now appoint 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."
Talks between the union and the National Gallery are due to be held at Acas next week.