Unison has accused care staff employers of effectively breaching minimum wage laws by failing to pay workers when they travel between home visits.
It says a survey of homecare staff reveals a 'pay scandal' and shows the workers are being exploited.
The local government union says three quarters of the staff who look after people at home are not being paid for travel time.
Its survey of more than 300 domiciliary care workers in England shows many are hundreds of pounds short each month as a result.
They spend almost a fifth of their working day travelling between people’s homes, and most are paid at or just above the minimum wage, the survey found.
The union is calling for travel time payment to become a contractual requirement and wants employers to put details of the reimbursement on pay slips.
Unison general secretary Christina McAnea said: 'Both care staff and those they look after are the victims of this pay scandal.
'Vulnerable people suffer when their already rationed care visits are cut short or delayed.
'Unison has been highlighting this exploitation for over a decade, yet the Government has responded with inaction and indifference.
'These appalling working practices must be tackled urgently if more people are to be encouraged to work in a sector desperately short of staff.
'Disabled and elderly people receiving care support will want an end to the exploitation of those looking after them. When it does, everyone will benefit.'
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