The Royal Mail has accused the Communication Workers' Union of talking "cobblers" as a 48-hour strike hits mail services.
Gordon Brown urged striking staff to get back to work
The war of words flared as Royal Mail boss Adam Crozier rubbished the main postal workers' union's suggestion that the firm treats its staff as "slaves".
Talking to BBC Radio 4's Today, Mr Crozier said the Royal Mail expected its workers to work their paid hours.
The row came as up to 130,000 CWU members strike over pay and pensions.
Postal workers began the 48-hour strike early on Monday after eight days of talks between the Royal Mail and CWU officials ended without a settlement on pay increases, pension benefits or job security.
CWU members also staged a 48-hour walkout last week in protest over the Royal Mail's modernisation's strategy which the union says will lead to a loss of 40,000 jobs.
The breakdown of talks also led to the union warning that a further week of "continuous disruption" to all Royal Mail services would begin next Monday if the dispute was not resolved.
The threat of rolling strike action raises fears that households and businesses could receive little or no mail during that period, and possibly for weeks to come as Royal Mail struggles to deal with a backlog of parcels and letters.
'Flexibility at stake'
Mr Crozier apologised for the inconveniences to Royal Mail customers the strikes were causing, but said it was important to settle the dispute in a way that the company "can survive and prosper and not for its own sake".
"Flexibility is the main issue at stake," he said.
Earlier, the CWU had said that some of its members would come to work at 6am, do a full day's work and at the end a manager could arbitrarily say whether they had worked hard enough for them to go home.
"I call that slavery," said Dave Ward, CWU General Secretary.
But Mr Crozier denied this kind of working practice ever happened.
Rather, he argued that the CWU was against the abolition of some of the so-called "Spanish practices" that were abandoned in the 1970s by most companies.
These include the freedom for a postal worker to go home before their shift has ended if they have completed their designated workload for the day, rather than help out where needed for the remainder of their working day.
Mr Crozier added: "We are simply asking people to work the 37 hours and 20 minutes they are getting paid to work and if they have to work longer, then of course they get paid overtime.
"For the union to say they can't accept that is frankly not a tenable position."
TUC general secretary Brendan Barber, who chaired the negotiations, said he would be keeping in close touch with both parties to "continue to seek to find a way forward".
"I am very disappointed that this phase of intensive talks has not resolved the dispute when real progress has been made and we have been edging towards an agreement," he added.
Royal Mail management and the CWU are at loggerheads over plans to cut workers' pension benefits in exchange for a 6.9% pay increase over two years.
"Royal Mail's proposals also include flexibility proposals that mean, among other things, that postal workers will not know what job they are doing from one day to the next," a union spokesman said.
The union has refused to accept these conditions, while Royal Mail described union demands as "unrealistic".
Planned 24-hour strikes
15 October from 1800BST: Mail sorting offices and airports
16 October from 0300BST: Deliveries and collection hubs
17 October from 1200BST: Royal Mail drivers
18 October from 1200BST: Merchandised data entry centres
18 October from 1400BST: Heathrow world distribution centre
Business groups have warned that the walkouts are causing immense disruption to the economy with reports suggesting the stoppages could cost the firm up to £260m.
It is feared that costs will rise over the longer term as customers switch to other postal suppliers and electronic forms of communication.
Postal economist Ian Senior blamed postal regulator Postcomm's attempts to keep prices well below Royal Mail's competitors in Europe for the company's woes.
He warned that the repeated industrial action was likely to push Royal Mail into a loss-making position and "hasten the long-term decline of the letter".