Homecare workers continue to receive less than the minimum wage because of the "scandal" of not being paid for the time spent travelling between visits, the Government is being told.
Unison said it believes the practice is widespread, while wage slips are often confusing so workers cannot check payments.
The union called on ministers to force employers to make pay calculations easier to understand.
Most homecare employees work in isolation and rarely see colleagues so it is difficult for them to compare their experiences, and firms seldom make amends and correct payments across their workforce if they fail to pay for travel time, said Unison.
General secretary Dave Prentis (pictured) said: "Homecare workers support the elderly and vulnerable across the UK, yet they continue to be paid below the minimum wage.
"The Government promised to act, but so far ministers have abjectly failed to help these low-paid workers.
"Homecare firms who only pay their staff for the face-to-face time they spend with their clients are guilty of law-breaking on a grand scale.
"The increasing use of 15-minute visits places untold pressure on homecare workers, yet their pay doesn't reflect the importance of the work they do."
Unison said a HM revenue and Customs report into six major care companies, commissioned by the Government a year ago, should be published.
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