The Association of Taxation Technicians (ATT) has warned that "severely outdated" mileage rates leave motorists out of pocket.
Providing that their employer uses HMRC-approved rates, employees can be reimbursed for business travel tax-free, according to ATT.).
For employees using their own car or van for business travel, these rates are 45p per mile for the first 10,000 miles, and 25p per mile after that.
ATT says the new rates should now be increased to accurately reflect the cost of using a vehicle for business.
Senga Prior, chair of the ATT Technical Steering Group, said:
"Freezing mileage rates for the last 12 years means employees are no longer fairly compensated for the real expenses incurred during their business travel.
"The current rates are severely outdated, meaning employees are bearing the financial burden of business travel on behalf of their employers."
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