New research finds the 950,000 UK workers employed through an agency, like Mr McCann, earn more than £400 a year less than those employed directly.
The Resolution Foundation figure includes wages as well as other benefits such as unclaimed holiday or sick pay, and deductions for uniforms.
That's more than the number of people on zero-hours contracts, according to the think-tank.
Lindsay Judge, senior policy analyst at the Resolution Foundation, said: "Agency work has been largely absent from recent discussions about the modern world of work, despite almost a million people being employed via an agency.
"As well as reforming poor regulation, the government should stamp out unlawful practices through tougher, targeted enforcement work."
The foundation is concerned that many agency staff are unaware of their contract rights, with £500m in unpaid holiday pay and 500,000 eligible for pension auto-enrolment who are not signed up to schemes. Its research, which is based pay on official ONS data, found that agency staff are more likely to be employed in lower wage roles across a wide range of professions.
Further, when the Resolution Foundation compared the pay of two similar individuals of the same age and background, working in the same kind of business, those employed by an agency received 22p an hour less than those directly employed. That adds up to £422 over the course of a year.
The think-tank wants contracts - so-called Pay Between Assignment - to be outlawed because they give firms a legal way to avoid equal pay obligations.