The six new dates are: Friday 9 December, Sunday 11 December, Wednesday 14 December, Thursday 15 December, Friday 23 December and Saturday 24 December.

This action will be in addition to the four days of strike action already announced for later this month, and on Thursday 1 December. 

A CWU spokesperson said: “The CWU want a negotiated settlement with Royal Mail Group and will continue to engage the company to that end.

“But those in charge of Royal Mail need to wake up and realise we won’t allow them to destroy the livelihoods of postal workers.”

The strikes will involve some 120,000 postal workers across the Royal Mail network.

Announcing the escalation yesterday evening amid the increasingly bitter dispute with Royal Mail management, CWU general secretary Dave Ward described Royal Mail’s proposals as “the biggest-ever attack on jobs and terms and conditions”, and said the situation was like “P&O on steroids”.

Parent group IDS announced its interim results yesterday, including its desire to scale back the current Universal Service Obligation at Royal Mail by ditching Saturday postal deliveries.

The net impact of industrial action in Royal Mail’s first-half trading came to £70m including lost revenue and strike preparation costs. 

Update: Comment from a Royal Mail spokesperson: “Our preference is for an agreement with the CWU, but the change we need is not optional. They should be focussed on a resolution to this dispute for their members and the long-term health of the business, rather than damaging strike action.”