Workers at the Royal Mail have voted to go on strike in protest against the company's move to replace its defined benefit pension scheme.
The Communication Workers Union (CWU) said 73% of its 110,000 members working at Royal Mail cast their votes, with 89% backing industrial action.
The CWU postal executive will meet later this week to decide its next move and agree on any potential strike dates, with the ballot valid for the next six months.
Sources told the Daily Mail that the union could decide to hold the strike on 24-25 November to coincide with the Black Friday weekend when many Britons start their Christmas shopping, potentially causing major disruption during the seasonal period.
The CWU is opposed to Royal Mail's plan to close its current defined benefit pension scheme in March 2018, while there are also disagreements over pay and working hours.
Any strike at Royal Mail would be the first since the company was controversially privatised in 2013.
"This ballot result is hugely significant and 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," said Terry Pullinger, deputy general secretary of the CWU.
"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 breakdown in relationships and a toxic environment where working together to solve difficult problems has become almost impossible.
"The managerial leadership has failed and should resign or be sacked."
Royal Mail said it was "very disappointed" by the results of the CWU vote and insisted that there are no grounds for industrial action, pointing to a legally-binding agreement it struck with the union four years ago concerning dispute resolution.
The company added that it was committed to continuing working towards a new pay and pension deal, including a salary increase of up to 5% depending on productivity improvements.
"Industrial action is damaging for our business," Royal Mail said in a statement. "It undermines the trust of our customers.
"National industrial action means the current offer from Royal Mail, including on pensions, will be taken off the table."