Posties could be set to strike over the Christmas period in a dispute over pay and working conditions.
Royal Mail workers who are members of the Communication Workers Union (CWU) have been been voting on whether to launch a campaign of action.
The result will be announced on Tuesday, with expectations of a yes vote among the union's 110,000 members involved in the row and could lead to a national strike.
The CWU says that the 503-year-old Royal Mail is not sticking to an agreement reached last year covering a wide range of issues, including plans to reduce the working week, as well as job security.
Industrial relations at the company, which was state-owned until 2015, have worsened this year, with widespread unofficial strikes breaking out virtually every week.
The Sunday Times reports that if members of CWU back calls for strike action it could lead to a mass walkout over the Christmas period or on Black Friday at the end of November.
Terry Pullinger, the CWU's deputy general secretary, said the union and its members were facing the ‘fight of our lives’.
‘We face an assault on our terms, conditions and national agreements like we have never seen before. I am confident our members will stand with the union and delivery a massive yes vote in what is the most important dispute the union has ever had,’ he said.
Shane O'Riordain, the Royal Mail's managing director of regulation and corporate affairs, said there were no grounds for industrial action.
He said: ‘We remain committed to open and constructive engagement with the CWU. We all want a successful and sustainable company that provides good quality jobs, fairness in workloads, and continues delivering a sustainable Universal Service.
‘No industrial action can be taken before the conclusion of our mediation period. We will continue to engage in the agreed dispute resolution processes set out in the Agenda for Growth.
‘Industrial action - or the threat of it - undermines the trust of our customers. It makes it harder for Royal Mail to pay for the existing industry-leading terms and conditions it provides.’
He said Royal Mail was honouring the 2018 agreement. Since it was put in place there have been two pay increases of 5 per cent and 2 per cent, the working week had been cut by an hour, and the company was working with the CWU to lobby the Government for a new pension scheme.
He added: ‘We have announced a transformation plan, through which we want to invest £1.8 billion over five years to return our UK business to growth. Hardly any UK companies are making that kind of investment right now.'