Members of the Communication Workers Union (CWU) will strike for 48 hours on Thursday and Friday and on 30 November and 1 December, and will also carry out single days of action on 9, 11, 14, 15 and 23 December and on Christmas Eve.

Royal Mail said it had made an offer to the CWU with “extensive improvements”, including an enhanced pay deal of up to 9% over 18 months, offering to develop a new profit-share scheme for employees and making voluntary redundancy terms more generous.

The CWU said the Royal Mail’s offer to postal workers would turn the service into “a gig economy-style parcel courier, reliant on casual labour”.

The union is calling for an improved 18-month pay deal, a guarantee of no compulsory redundancies and an “alternative business strategy” that it says would allow Royal Mail to grow.

The CWU’s general secretary, Dave Ward, said: “We are disappointed that instead of reaching a compromise to avoid major disruption, Royal Mail have chosen to pursue such an aggressive strategy.

“We will not accept that 115,000 Royal Mail workers – the people who kept us connected during the pandemic, and made millions in profit for bosses and shareholders – take such a devastating blow to their livelihoods.

“These proposals spell the end of Royal Mail as we know it, and its degradation from a national institution into an unreliable, Uber-style gig economy company … We urge every member of the public to stand with their postie, and back them like never before.”

The government has rejected a request from Royal Mail to stop delivering letters on Saturdays, but the company says it faces financial ruin unless the terms of service are changed.

In October the company, which reported a £219m loss in the six months to September, said it could have to cut up to 10,000 jobs by next August.

Simon Thompson, Royal Mail’s chief executive, said: “Talks have lasted for seven months and we have made numerous improvements and two pay offers, which would now see up to a 9% pay increase over 18 months alongside a host of other enhancements. This is our best and final offer.

“Negotiations involve give and take, but it appears that the CWU’s approach is to just take. We want to reach a deal, but time is running out for the CWU to change their position and avoid further damaging strike action tomorrow.

“The strikes have already added £100m to Royal Mail’s losses so far this year. In a materially loss-making company, with every additional day of strike action we are facing the difficult choice of whether we spend our money on pay and protecting jobs, or on the cost of strikes.”