Agency workers will stage protests outside call centres, claiming a loophole in the law is costing some more than £500 a month because they are being denied the right to equal pay.
The Communication Workers Union said that, under the Agency Workers Directive, all agency workers should receive the same rates as other workers after 12 weeks in a job.
But the union said many agency workers are signing away their rights without realising what they are doing.
General secretary Billy Hayes said: "These contracts are legal, but in the same way that the legal tax arrangements of Starbucks, Amazon and many celebrities are morally wrong, we believe these contracts fly in the face of fairness.
"Both agencies and hirers are at fault for choosing to use these contracts which sign away workers' rights to equal pay, rather than sticking to the spirit of the new legislation which had equal pay at its heart.
"Agency staff earning less than the living wage are losing out to the tune of £500 or more a month. They're entitled to equal pay but are being exploited. This loophole should be closed."
A study by the union found that agency workers in call centres were losing between £150 and over £500 a month.
Grahame Smith, general secretary of the Scottish TUC, said: "The STUC strongly supports the CWU's day of action.
"It is as predictable as it is disgraceful that employers have so quickly ferreted out loopholes in the 2011 legislation to avoid paying fair wages to hardworking employees already struggling with the insecurities of temporary employment."