Mr Lynch said: “We need a summer of solidarity, and a spring of solidarity if it needs to go through next year. The CWU, Unite, GMB, RMT and the others, we have to call on the entire movement […] to come into this action, to get members motivated and call them to the flag and vote yes for a wave of industrial action across the UK and internationally if that is what it takes, because we need to redress the balance in society.
“And not be dictated to by people from Eton and Harrow, telling us we have to give up our wages and give up our place. We are not going to have it.”
Royal Mail is grappling with the disruption as its executives attempt to force through an overhaul in its working practices.
The company, privatised by the former business secretary Vince Cable in October 2013, is facing the prospect of being split in two after losing ground to Amazon over parcel deliveries.
Management are also considering ripping up an agreement with unions that prevents them from outsourcing work or imposing compulsory redundancies.
Meanwhile, the board is bracing for a national security investigation after it was revealed that billionaire investor known as the “Czech sphinx” prepares to take control of more than a quarter of the business.
Kwasi Kwarteng, the Business Secretary, has told the company that he “reasonably expects” Daniel Kretinsky to increase his shareholding to more than 25pc, a move that would trigger an investigation that could ultimately mean the billionaire is forced to sell down his stake.
His estimated net worth of about $4.1bn (£3bn) is supported by his ownership of Energeticky a Prumyslovy Holding (EPH), the biggest energy group in Central Europe.
The tycoon is a major investor in West Ham United football club and holds stakes in French newspaper Le Monde, as well as retail giants Sainsbury’s and Macy’s.
The RMT said trade unions support each other where they can on the principle of mutual solidarity for better wages and against job cuts.
The CWU declined to comment.