Unison general secretary Dave Prentis said more than 200,000 care workers were receiving illegal wages.
Unison said councils across England and Wales were not insisting that homecare companies pay staff their travel time, leading to hourly rates below the statutory £6.70.
The union said its research showed that three out of four local authorities in England, and even more in Wales, do not stipulate in their contracts that employees should be paid for travelling between appointments.
"More councils might now be insisting that homecare contracts ensure payment for travel time, but there's still too many that don't," said Mr Prentis.
"This shows just how little local authorities value care staff who do such a vital job looking after the elderly and disabled.
"Councils shouldn't be awarding contracts to firms without ensuring they're prepared to pay travel time.
"The Government and councils must act now to put a stop to the shocking treatment of this dedicated group of employees."
The report was published ahead of a debate in Parliament on the issue.