A senior boss from the Communication Workers Union (CWU) has said he would rather "smash Royal Mail to bits" than back down over a dispute over pensions.
Terry Pullinger, who is deputy general secretary of the CWU, made the comments on Thursday during a rally for postal workers in central London.
The union has been embroiled in a long-running dispute with Royal Mail over proposed changes to pensions for workers.
Mr Pullinger told crowds on Thursday: "They'd better listen, because if they don’t make significant moves, then this will be a fight to the end because I’ve said it before, this is being wrecked this industry, over a long period."
He added, in a video which was also posted to the union's Facebook page: "Well we’re not going to put up with that. Bring it on, because if that’s what they’re determined to do we’d rather smash it to bits than hand it over to them to make all their money."
Postal workers marched across London from the TUC Headquarters in Bloomsbury to the Royal Mail’s Mount Pleasant Delivery Office on Thursday in protest against moving workers to a new pension agreement.
Royal Mail last week won a High Court injunction blocking a planned 48-hour strike by postal staff after claiming it was “unlawful” because the union had failed to follow previously agreed upon dispute resolution procedures.
But the CWU has called its actions "underhanded" and said the march is the next phase in its fight for what it thinks are fair terms for workers.
Mr Pullinger said: "If Royal Mail don’t move, I’m telling you now, we will not back off in this dispute. Whether it’s six months, or whether it’s years."
Royal Mail says the changes to pensions for some workers will help curb its ballooning deficit. It has warned that its top-up payment obligation would more than double to £1.26bn next year without any pension reform.
The company is planning to move its workers from the current final salary scheme to a form of defined benefit scheme, which will affect new members from April next year.
But unions have warned that it will leave many workers worse off, and despite 18 months of talks they have failed to reach an agreement with the firm.
Members of the CWU had voted overwhelmingly to take industrial action when the results of a ballot were announced earlier this month.
A spokesman for Royal Mail said it was "very committed to working closely with the CWU" in order to reach an agreement.
"We want to use the mediation process under the dispute resolution procedures in the Agenda for Growth to do just that," he said.