More than 130,000 postal workers are to be balloted on strike action in a row over pay.
Royal Mail recently asked for a stamp price rise to cut its losses
The Communication Workers' Union (CWU) rejected a 2.5% pay rise offer and said it would begin balloting staff later this month.
The CWU said Royal Mail's business plan would lead to 40,000 job losses, pay cuts, mail centres and delivery office closures and threaten pensions.
The Royal Mail said it was a fair offer in a "competitive" market.
It added that the recent opening up of the UK's postal sector meant it had lost business to rivals.
The CWU has been calling for a 27% pay rise over five years, which it says would bring general worker pay in line with the national average.
But Royal Mail said such a pay deal would be "madness", adding it was "disappointed" by the union's decision to hold a strike ballot.
"For the union to demand a 27% increase in pay and a shorter working week - at a cost of £1bn - is madness given Royal Mail already pays its people 25% more than its rivals pay and is 40% less productive," a Royal Mail spokesman said.
"To call for strike action when we are already rapidly losing market share to those rivals is also insane and shows that the union has no understanding of the very tough commercial environment we are operating in."
However, the CWU argued that the Royal Mail had reneged on a deal agreed last year to limit the impact of competition and automation.
"Royal Mail has abandoned our agreed approach in favour of a short sighted Business Plan that amounts to a cost-cutting frenzy, reductions in pay and a defeatist attitude towards competition," the CWU said.
Should the CWU vote back action, it could trigger the first national post walkout in 10 years.
Ballot papers will be sent out to CWU members on 22 May and the result of the vote will be announced on 7 June.