Royal Mail has accused the union leading strikes by its workforce of allowing violence on its picket lines.
The Communication Workers Union is involved in industrial action in the lead-up to Christmas as part of a long-running dispute over jobs, pay and working conditions.
Royal Mail said 35 picket line incidents had been reported to police in Scotland, Northern Ireland, West Midlands, Lancashire, London, Kent, Norfolk, Devon, Cornwall and Gwent.
Britain is set to be crippled by industrial action in December, with strikes set to take place every day until Christmas, affecting many sectors.
The news broke as CWU shared a tweet on Thursday with a video purporting to show Royal Mail’s chief executive, Simon Thompson, being coached during an interview on BBC Breakfast.
The CWU claims the video footage shows the silhouette of a staffer writing answers on a board reflected in the glass of a picture during the interview on Wednesday.
It tweeted: “We have spotted something during Simon Thompson’s car crash interview on the BBC this morning.
“Having zoomed in, someone is in the room writing answers on a board for him.
“Pack your bags, Thompson. It’s time to go.”
A Royal Mail representative said the alleged poor picket line behaviour had “no place in any civilised workplace”.
“The direct intervention of the CWU's general secretary is needed to end this culture of intimidation on the picket line, including many incidents by CWU representatives.
“More than 10,000 frontline employees chose to work on each of the last two days of strike action, and with numbers increasing with each passing day of industrial action, they should feel confident that they can do so safely and without the risk of harm. We have zero tolerance for bullying or harassment.
“We will continue to support any employee who works on a strike day. They can be assured that we will investigate every incident and allegation and take swift action to discipline the individuals involved appropriately.”
A CWU representative called the allegations “as embarrassing as they are untrue”.
“For a chief executive who spends his every waking hour intimidating, gaslighting and mocking our members online to have the front to target others for wholly legal and friendly picketing is beyond a joke.
“Prior to every round of strike action, the union has reminded members to turn out in record numbers whilst being peaceful and respectful. Our members and reps have done us proud.
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“Our members want to go back to work and save Christmas for 32 million households in the UK, but they will not bow down to a culture of intimidation.”
More than 115,000 workers at Royal Mail first walked out in August after rejecting a 5.5 per cent pay rise for CWU-grade workers, the biggest increase workers had been offered in years.
The CWU said its members were taking the action for a “dignified, proper pay rise” after they voted in favour by 97.6 per cent in a ballot.
The union has staged regular action since.
More strikes are planned in the coming weeks, including on Christmas Eve.
Britain will suffer strikes throughout December as discontent spreads about pay, which many workers claim is failing to keep pace with inflation, which is at a 41-year high of 11.1 per cent.
Rail workers, teachers, nurses and driving instructors are among those staging industrial action over the festive period.
Civil servants, including Border Force officers, Passport Office staff and National Highways employees have also voted to strike but have yet to announce dates. The action is expected to take place before Christmas.
On Wednesday, Unite and GMB unions announced ambulance service workers also backed action, which could take place in December.
The announcement came a day after Unison said thousands of health workers, including ambulance staff, had voted to strike.