Support staff at a Sheffield school whose jobs were handed to a contractor are to strike for the living wage, the GMB union said today.
Friday's strike will create another headache for privateer Carillion, which is at the centre of the nationwide construction industry blacklisting scandal.
The 24 support staff are school cleaners.
Sheffield City Council recently opted to become a living wage employer and will pay its directly-employed staff at least £7.45 an hour from April this year - as well as vowing to "name and shame" private contractors who refuse to implement the living wage.
But Carillion, which took over most of the jobs two years ago, has refused to just that.
GMB regional officer Peter Davies said: "Most of the city's lowest-paid workers in schools and other public services have been outsourced to contractors over successive administrations.
"Where this has happened, these Sheffield City Council contractors are refusing to honour the rise to £7.45 per hour for the city's lowest-paid employees.
"We have a ridiculous situation at Firth Park community college and other schools where workers work directly for the council in the kitchens for half of their job and then for Carillion later as cleaners. Carillion pays these workers over £1 an hour less."
Mr Davies added: "Council contractors were all quick to follow the city pay freeze but when there is something on the table for the lowest-paid they suddenly run for the shadows.
"The only way these workers will get the living wage is to fight for it through a sustained industrial action campaign like the Carillion workers are doing."