The group of 27 artists and art lecturers, many of who worked at the gallery for decades, allege they were dismissed in October 2017 without consultation or benefits as they were classed as self-employed.
Lawyers for the claimants will argue in a ten-day employment tribunal hearing, which began on Monday morning, that the art educators should be recognised as employees and afforded corresponding rights.
According to the legal team, the hearing will also decide a new point of law - whether workers are entitled to collective consultation prior to termination of a contract - not considered by previous gig economy cases.
Gallery managers deny that the workers were dismissed, insisting the majority are still in their posts and that all were consulted on changes to their contracts.
While most recent test cases on employment status have related to private sector firms such as Uber, Deliveroo and Hermes, the educators say that their case demonstrates that exploitation also exists in the public sector.
Speaking at a meeting with the claimants to discuss the upcoming employment tribunal, Mr Corbyn said: “I'm here to support the campaign because I'm very concerned about the number of people in this country who are frankly 'bogusly' self-employed.
“In reality they are employees of big companies or organisations all over the country. I'm determined that a future government, a Labour government, will do things very differently including ending bogus self employment.”
Ms Creasy added: “The decisions of the National Gallery and the ramifications of this case go far beyond what happens to the 27 individuals who are taking on this class action.
“It's really the test about whether our public services are behaving in an ethical fashion.
"Not just in the way that they pay people, but how they treat people when they want to make changes - and what this means in the world today when more and more people are being classed as 'self-employed'."
The group of 27 educators claim that in total they provided more than 500 years of service to the National Gallery. They welcomed school tours and members of the public to the institution and gave the daily programme of talks, courses and workshops about the works of art on display, as well as delivering learning sessions in schools and hospitals.
While the National Gallery classed them as self-employed, the educators said that they worked regularly, were paid through the company payroll, taxed at source and were required to attend training sessions and appraisals.
When the gallery moved staff from temporary to permanent contracts, the educators claim they were dismissed with just eight permanent contracts offered to the group on reduced salary and terms.
Steven Barrett, a claimant who worked as an educator at the gallery for 13 years, said: “Everything we did was for the National Gallery and not ourselves - we weren't in any way self-employed.
“But it’s not just about us, so many people seem to suffering similar abuse from 'bogus' self-employment. And our case has the potential to be transformative as it is the first time this issue has been located within the public sector.“
Marie van der Zyl, partner at Gordon Dadds LLP, the groups's legal representative, said: “Individuals working in the arts are in need of certainty surrounding their employment rights and it’s essential to ensure they are categorised correctly.”
The claimants have set up a page on the crowdfunding website Crowd Justice to raise funds for their legal claim, gathering £70,000 so far.
Marie-Therese Ross, a claimant who worked for the National Gallery for 24 years, said: “This isn’t just about us. Our case highlights the exploitation of precarious workers across the arts and beyond.
“We are standing up for fair employment rights and calling for our public arts organisations to value the expertise and experience at the heart of their education programmes.”
A spokesperson from the National Gallery said: “The National Gallery has been issued with a number of different claims from a group of freelance workers who have been providing a range of different services for the Gallery (and other museums and galleries across London) on an ad hoc basis for a number of years.
"The Gallery has not dismissed anyone as part of this process. The majority of the people involved are still providing these services to us on the same basis as previously, whilst others involved in these claims have already accepted either employment or new contracts with us.
"It is our understanding that the claims have arisen out of the National Gallery's choice as an ethical employer to change from offering ad hoc work to offering more secure employment, with additional pension and worker benefits.
“This change also reflects our strategy to develop gallery programmes to reach new audiences and to enhance our engagement with visitors.
”The entire group was consulted for their views about the change - both together and individually - for a period of three months between October 2017 and January 2018.
“Jobs were offered to all of our existing freelance service providers last year. We still have vacancies available, although unfortunately not all members of the group have expressed an interest in these."
The National Gallery said that the case “should not be likened to the gig economy debate” and that the situation at the gallery was “the opposite”.
“We have taken a deliberate choice to move towards a model that offers people secure employment, with additional pension and worker benefits,” the spokesperson added.
The Central London employment tribunal is expected to rule on the case on 7 December.