The changes include the removal of paid breaks and shorter breaks for visitors and sales assistants working at the National Maritime Museum and the Royal Observatory Greenwich.
Staff calculated that by reducing breaks by 40 minutes a day, they will be working three extra weeks a year and as a result these changes have become a sticking point with Prospect members.
Since the changes were made management have been working with Prospect to address some of the other concerns raised including the hours shifts fall within and seating for staff on shift.
Management has also committed to start working towards paying the London Living Wage in the next four years
Prospect negotiations executive Sharon Brown said: “Prospect has been working hard to ensure that our members concerns are addressed. We have achieved some changes which have been beneficial to workers at the museum.
“However, members are still hopeful that management will be able to change their policy on breaks and make real progress towards achieving the London Living Wage.
“This action is not an attempt to stop people going to the museum. The aim is to highlight how staff have been affected by these changes and to try to find a resolution.”