RoadChef, the UK's third-largest motorway services operator, is planning to cut sick pay, holiday pay, overtime rates and pension benefits for 2,500 staff from next month.
The GMB union reacted angrily to the plans and advised its members not to sign the new contracts, which are due to come into effect on 1 November.
It also threatened the company, owned by Israeli investment firm Delek Real Estate, with court action "if necessary".
Under RoadChef's plans, the sick pay scheme is being discontinued and meal breaks will go unpaid. The GMB added that, under the new conditions, overtime would not be paid until after 48 hours of work, up from 39 hours.
The union also claimed that workers at the group's 29 stations may also lose their permanent employment status.
Mick Rix, the GMB's national organiser, claimed that previous private equity owners Nikko Principle Investment and Vision Capital "loaded RoadChef with debt".
"RoadChef is a prime specimen of what the Prime Minister labelled ‘the age of irresponsibility," he said.
"What is really galling is that the private equity firms that stripped out the assets of this company have walked away scot-free and left ordinary workers to pay the bills.
"A debt burden of £425m is a very heavy load for 2,500 mainly minimum wage workers to bear. The interest payments to service the debts are well above the annual wage bill."
RoadChef insisted the GMB was being "unfair" and said that, while the company had proposed new terms and conditions, staff who signed the new contract would receive a 10% pay rise in hourly pay.
The GMB countered that, even with the pay rise, staff would be worse off.
By Daniel Thomas
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