Postal staff are returning to work following a 48-hour strike, but a fresh set of 24 hour walkouts is being threatened as soon as next week.
Gordon Brown urged striking staff to get back to work
The war of words between the Royal Mail and Communication Workers Union (CWU) has not raised hope of a solution.
The CWU's David Ward described Royal Mail's work practices as like "slavery", which the company's boss Adam Crozier said was "cobblers".
The CWU will meet on Wednesday to consider taking further action.
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: Manual data entry centres
18 October from 1400BST: Heathrow world distribution centre
Next week, it is proposing to have one department at a time walk out for 24 hours, maximising disruption while only making members forfeit one day's pay.
Up to 130,000 CWU members have been striking in a protest over pay, working conditions and Royal Mail's modernisation's strategy, which the union says will lead to a loss of 40,000 jobs.
Mr Crozier apologised for the inconvenience caused to Royal Mail customers by the strikes 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 then a manager could arbitrarily say whether they had worked hard enough 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.
'Not a tenable position'
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.