Mr Lynch said: “We need a Summer of Solidarity, and a Spring of Solidarity if this is to continue next year. The CWU, Unite, GMB, RMT and others, we need to appeal to the whole movement […] to get in on this action, to motivate members and call them to the flag and vote yes for a wave of industrial action across the UK and internationally if that’s what it takes, because we must restore balance in society.

“And not being dictated to by people in Eton and Harrow telling us we have to give up our pay and give up our place. We’re not going to have it.”

Royal Mail is grappling with the disruption as its executives try to force an overhaul of its working practices.

The company, privatized by former business secretary Vince Cable in October 2013, is at risk of being split in two after losing ground to Amazon on parcel deliveries.

Management is also considering tearing up an agreement with unions that prevents them from outsourcing work or imposing forced layoffs.

Meanwhile, the board is preparing for a national security investigation after it is revealed that the billionaire investor known as the ‘Czech sphinx’ is preparing to take control of more than a quarter of the company.

Kwasi Kwarteng, the business secretary, told the company he “reasonably expects” Daniel Kretinsky to increase his stake to more than 25%, a move that would trigger an investigation that could ultimately mean the billionaire is forced to sell its stake.

His estimated net worth of around $4.1bn (£3bn) is backed by his ownership of Energeticky a Prumyslovy Holding (EPH), Central Europe‘s largest energy group.

The tycoon is a major investor in football club West Ham United and has stakes in French newspaper Le Monde, as well as retail giants Sainsbury’s and Macy’s.

The RMT said unions supported each other where they could on the principle of mutual solidarity for better wages and against job cuts.

The CWU declined to comment.