Royal Mail's plan to close its defined benefit pension scheme next year will be met with the "strongest possible opposition", including a ballot for industrial action, says a union.
The Communication Workers Union (CWU) issued the threat moments after the company confirmed aims to halt the scheme from March next year on cost grounds, following a consultation with the plan's 90,000 members.
It claimed Royal Mail's proposed replacement would result in its members facing "poverty" in retirement, losing up to a third of their current future pension value.
The company's statement said: "Royal Mail has reviewed the consultation feedback received from members and its unions, the CWU and Unite/CMA.
"This includes a proposal put forward by the CWU. Sustainability, affordability and security are the principles the Company employed when reviewing the consultation feedback.
"The Plan is currently in surplus but we expect the surplus will run out in 2018. The Company's annual pension contributions are currently around £400m.
"If no changes are made, the contributions could more than double to over £1bn in 2018.
"We have concluded that there is no affordable solution to keeping the Plan open in its current form.
"Therefore, the company has come to the decision that the Plan will close to future accrual on 31 March 2018, subject to Trustee approval."
The CWU accused Royal Mail of offering an "inferior" alternative, saying its own suggestion of a defined benefit Wage in Retirement scheme would have given it a "credible, cost efficient and last pension solution".
Its acting deputy general secretary, Ray Ellis, said: "Although Royal Mail's own consultation exercise revealed massive opposition to the closure plan, the company has decided to ignore the views of its workforce and proceed with closure without consent."
He added: "We will not stand by and watch the company abandon the pension promises it made at the time of privatisation which threatens our members with massive cuts to their future pension benefits and insecurity and poverty in retirement."
Unite said it could not rule out an industrial action ballot with no resolution.
Royal Mail said it would look to work with the unions to find a "sustainable and affordable solution". Its share price closed 1% higher on the FTSE 100.