The majority of Royal Mail workers has voted in favour of industrial action over a pension and pay dispute.
According to the Communication Workers Union (CWU) the vote had a 73.7 per cent turnout with 89 per cent backing a strike.
If it goes forward Royal Mail will face its first nationwide strike since it was privatised four years ago, in a move which could threaten pre-Christmas deliveries.
CWU officials will meet later this week to determine the next steps and any potential strike dates but it has been suggested workers will walk out on so-called "Black Friday" in late November - when Christmas shopping begins and many retailers offer large discounts.
But Royal Mail says the union cannot take industrial action until contractual dispute resolution procedures - which escalate to independent external mediation expected to take place close to Christmas - are completed.
The CWU said the vote was sparked by Royal Mail's “attack on the pension rights of hard-working postmen and women" as well as its the "refusal" to engage over pay, working hours, future job security and the need to improve and grow the service.
Royal Mail had a deadline which ended on 6 September to solve its long running pension dispute with workers.
Earlier this year, the UK postal operator announced it would be closing Royal Mail Pension Plan, its current defined benefit (DB) scheme, to future accruals from next year.
Following union opposition the company is offering members the choice of joining either a DB cash balance scheme or a defined contribution scheme.
From 1 April 2018, the DB cash balance scheme would provide members with a guaranteed lump sum at retirement.
But according to Terry Pullinger, CWU’s deputy general secretary, this ballot result “demonstrates a strength of feeling that can only be translated as a massive vote of no confidence in the managerial leadership of the Royal Mail Group and the direction that they advocate”.
He said: “Any sense of vocational spirit and working together with management has been lost in a climate of fear and insecurity.
“This massive failure in trust has created a break down in relationships and a toxic environment where working together to solve difficult problems has become almost impossible.”
In the meantime, Royal Mail has said that no industrial action is planned and that it is “business as usual”.
The company also added it is “committed to further talks as a matter of urgency, to reach agreement with the CWU”.
Nathan Long, senior pension analyst at Hargreaves Lansdown, said that Royal Mail’s decision to close their DB scheme is in line with many employers’ attempts to cap the uncertainty of future pension payments.
He said: “Two replacement schemes have been tabled. Both offer generous pension contributions in comparison to most employers.
“Whilst the pension benefits are undoubtedly being watered down, a first-class retirement is still available for staff who maintain membership throughout their working life.”