About 400 postmen and women are leaving Royal Mail per week, according to the Communication Workers Union (CWU), who claims this number has gone up since the company introduced a new delivery system earlier this year.

The CWU says the firm had been losing about 330 postal staff a week from a workforce of 83,000 before it rolled out DM26, which reorganised routes to improve delivery performance.

The union claims the new system is not working in hundreds of offices, and BBC Your Voice has heard from staff warning of a "nightmare" Christmas ahead as the number of parcels increases.

Royal Mail says the reforms are part of its plan to deliver a more "efficient and financially sustainable" service.

Over the last year we've been in touch with more than 30 postal staff in all four nations of the UK, as they have switched over to DM26.

It allows for second class post to only be delivered every other weekday, and is supposed to guarantee first class mail and parcels are delivered every day, by putting workers into groups of three and changing the walking pattern of rounds.

But posties speaking to us anonymously say parcels are still being prioritised, letters left behind, and time wasted in sorting and dividing mail into first and second class.

"I didn't take any first class mail out on my last shift because I was on a road that's been behind for weeks," a worker in the West Midlands told me. "But I'm sure a manager would have ticked a box to say all first class mail went out." A Royal Mail spokesperson said the company always worked to ensure all mail - both letters and parcels - were delivered on time.

Some postal workers we've spoken to say the new model is also increasing their workload.

We asked eight workers in different areas to track their average step counts. Since the switchover all their steps increased by a quarter or even a third, often up to around 25,000 steps a day. Several fear this Christmas will be a "nightmare".

"We all think this Christmas will be the worst ever, for us and our customers," said one postman in the East Midlands.

Bryn Dymott delivered post in the village of Wootton in Bedfordshire for nine years. For the most part he loved the job. But he left the company in July saying he couldn't face another "shambolic" Christmas.

"People put things in the post for them to be delivered, not for them to be delayed because Royal Mail hasn't got the systems in place and enough people to get them delivered." Bryn says he wanted to speak out after leaving the company because he was sick of the fact that posties aren't able to say what's really happening.

He says the company is "proving it's impossible to have four people's work done by three people. It shouldn't really come as a surprise." So far DM26 has been introduced in about 1,000 of Royal Mail's 1,200 delivery offices.

Now the CWU has told its members to pause further roll-out of the new system until a plan can be agreed with Royal Mail to address problems.

The union says that while it is operating successfully in around 500 offices, it has run into significant problems in hundreds of others.

"Put simply, if a boat is already taking on water, adding more passengers is unlikely to solve the problem," says Martin Walsh of the CWU.

The union believes most Royal Mail workers leaving the company are new starters on part-time contracts, but also says there has been an increase in staff on legacy contracts departing.

Royal Mail, on the other hand, says the new system is on track to be up and running in all offices by Christmas.

It says the roll-out is 90% complete and that it is now delivering to "95% of addresses every day across the UK, up from 88% before the roll-out. The roll-out continues across our network and we expect quality of service to improve as deployment progresses." The company says it is hiring, as usual, an additional 22,000 temporary workers over the Christmas period.

Royal Mail has battled with increased competition and falling demand for letter deliveries in recent years, and last week announced plans to cut 2,500 head office and other supporting roles by the end of 2027.

The company has also faced years of criticism from politicians and the public over slow letter deliveries - something which has continued since the new system has been rolled out.

The regulator Ofcom has fined Royal Mail more than £37m in the last three years for missing delivery targets.

But some MPs think it is time for a new approach, and a group of them will meet the regulator later on Wednesday.

"We need to think outside the box," says Dave Robertson, Labour MP for Lichfield, Burntwood and the Villages in Staffordshire.

"We need to bring as many people to the table on this to make sure we come up with a long-term solution because what's been tried up to now clearly isn't working."

An Ofcom spokesperson said not only has it issued fines to Royal Mail in recent years but it has also demanded an improvement plan and reformed the obligations imposed on the company, adding: "We will continue to hold it to account using the full extent of the powers available to us."

Postal Services Minister Kate Dearden said Royal Mail's improvement plan was "a step in the right direction, but it must deliver tangible improvements for customers. We will continue to hold them to account on their performance." Get our flagship newsletter with all the headlines you need to start the day.