Workers at the Ritzy cinema in Brixton, south London, will go on strike again on Saturday as a dispute over pay enters its second year.

Members of the Bectu union will walk out for 24 hours. The row has since spread to several other Picturehouse cinemas in London and Brighton.

The workers want higher wages, sick pay and the reinstatement of several union representatives who were sacked this year after industrial action.

Sophie Lowe, one of the striking Ritzy workers, said: “One year on from the start of this dispute, we are planning a substantial escalation in the campaign.”

There will be nine strikes during the British Film Institute’s London film festival next month. The action will include a demonstration at Leicester Square on 4 October – the first day of the festival – involving staff from the Crouch End, Hackney and East Dulwich Picturehouses, as well as Picturehouse Central and the Ritzy.

During the festival, staff at the Hackney and Central cinemas will strike from 5pm on 6-8 October and from 11-15 October.

The BFI said it supported the Picturehouse workers’ call for payment of the London living wage, which it paid its own employees.

“The BFI is disappointed that the London Film Festival has become associated with this industrial action. The current plan is that LFF screenings will go ahead as published at both Picturehouse Central and Hackney Picturehouse.”

The campaign has won the support of high-profile figures including actor Sir Ian McKellen and director Ken Loach. They are among 25 actors and screenwriters who have urged the public to boycott Picturehouse cinemas and their parent company, Cineworld.

Loach, a Bectu member, said: “Picturehouse is owned by Cineworld, which is a big multinational corporation. They make fortunes. The idea that they pay starvation wages because they can get people who are desperate for work is absolutely shocking. Victory to the Picturehouse striker, no doubt.”

The struggle of Picturehouse staff has also garnered political support with the shadow chancellor, John McDonnell, and other Labour MPs writing a letter in the Guardian expressing concern over treatment of trade unionists.

“Any suspensions or sackings intended to intimidate trade union members would be a matter of deep concern for the whole labour movement. No trade unionist or Labour party member can tolerate the deliberate intimidation of trade union activists for pursuing their legitimate business,” they wrote.

Picturehouse has created the Picturehouse Staff Forum, described as a “fake union” by Bectu members. They claim it has been set up by Picturehouse specifically to block Bectu from being able to negotiate on behalf of its members at other cinemas.

A Picturehouse statement said: “Paying our staff a fair wage is hugely important to us. Staff at the Ritzy are paid £9.10 [an hour] – equivalent to £9.70 when working an eight-hour shift as we choose to pay for breaks.

“We also offer statutory sick pay, maternity/paternity pay and 28 days’ paid holiday among other competitive benefits. This makes Picturehouse Cinemas one of the highest-paying employers in the cinema industry.”

The London living wage is £9.75 an hour – 65p more than what Picturehouse staff are paid. This is calculated by the Living Wage Foundation and has led other cinema chains such as Curzon to agree to pay their staff this rate in 2014 amid a similar dispute.

Cineworld reported a pre-tax profit of £98.2m for 2016 – slightly lower than the previous 12 months.

Naomi Taylor, a Bectu official, told the Guardian: “Cineworld owns Picturehouse and Cineworld’s chief executive earned £2.5m just last year. Picturehouse can afford to pay their staff a living wage – they just choose not to.”

About 70% of Cineworld’s UK front-of-house staff, including 700 at Picturehouse are on zero-hours contracts without benefits such as parental leave or sick pay.