Workers at London’s National Gallery have entered a 10-day strike to protest the alleged sacking of union representative Candy Udwin and plans for privatisation.

With privatisation affecting around 400 of the gallery’s 600 staff, members of the Public Commercial Services (PCS) union are one-day into a 10-day strike, which will run until 4 June. A rally at Trafalgar Square is also planned for 30 May.

The PCS says that Udwin was dismissed following her suspension on the eve of Gallery workers' initial strike in February.

“The case against her was that she shared information about the use of a private security firm with her full-time union official and asked him to take up the matter with the gallery,” said the PCS in a statement. “We believe this is a clear case of victimisation and a direct attempt to undermine our union and we will fight it every step of the way.”

A petition calling for the reinstatement of Udwin has been signed by more than 21,000 people, including high-profile figures such as Russell Brand, MP John McDonnell and film director Ken Loach.

In a statement, the National Gallery said it hoped to keep most of the gallery open for business during the strike, but added that more than 5,000 school children have had trips to the gallery cancelled because of the strike action.

“The PCS opposes the introduction of a new roster for some visitor-facing and security staff, which would enable us to operate more flexibly,” said a spokesperson for the gallery.

“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.”