A fleet consultancy claims that current tax-free mileage reimbursement rates do not leave employees who use their own cars on business out of pocket.
The claim comes just days after the Community District Nursing Association said its members were effectively subsidising the NHS by using their own cars for work at a loss.
Fleet Audits said these claims are not borne out in reality.
Next Wednesday (March 12) the Chancellor of the Exchequer Alistair Darling is expected to amend the Authorised Mileage Allowance Payments (AMAP) system.
AMAP allow employees to reclaim a fixed level of costs of business travel in their own vehicles.
The current rates are 40p per mile for the first 10,000 business miles per year and 25p per mile thereafter.
Last minute lobbying is now at fever pitch as those who want an increase in the rates vie for the Chancellor's ear with those who want to see the rates reduced.
Fleet Audits claims that for the overwhelming majority of people claiming AMAP payments, the cost of running their vehicle is less than 40p per mile.
A spokesman said: “Current rates are more than adequate to reimburse the overwhelming majority of employees in the AMAP community."