The union representing Royal Mail's frontline staff is on the verge on announcing new strike dates, Sky News understands, should a final push to end their long-running dispute fail.

A Communication Workers Union (CWU) source said talks at the conciliation service Acas were scheduled for Thursday but fresh walkouts could be called the following day should substantial progress not be achieved.

There is deep frustration, on both sides, following 11 months of stop-start negotiations over pay, jobs and conditions for the 112,000-strong workforce.

There were 18 strike dates called last year and 2023 has seen the union and Royal Mail attempt to make progress at Acas.

This month, former TUC general secretary Sir Brendan Barber joined the effort to deliver peace.

Royal Mail is expecting the union to respond to its latest set of proposals on Thursday and the union source expressed hope that the latest meeting could build on some "positive" elements but did not hold out much hope.

Sky News understands that should the union call new strike dates, the loss-making company's board would not seek a drastic response by declaring an insolvency, as has been reported, at least in the immediate future.

The Guardian said on Monday that Royal Mail could seek to put the part of the company responsible for delivering letters six days a week into a form of administration without a deal.

The company has said that while it remains committed to a settlement, it must be affordable and in line with its plans to make Royal Mail more competitive including over Sunday working.

It has repeatedly said that job security would be increasingly imperilled the longer the dispute goes on.

The union has described the company's self-dubbed modernisation plans as an 'Uberisation', declaring that it would turn Royal Mail into a gig economy-style employer.

It secured a fresh mandate for industrial action in mid-February and would have to give seven days' notice of any fresh walkouts.

Neither the CWU or Royal Mail would comment publicly on the possibility of administration.

Such a move, which would need government approval, could potentially see the taxpayer forced to guarantee the company's Universal Service Obligation (USO), under which it has to deliver mail to every address in the country between Monday and Saturday.

It would be similar in nature to the appointment of a special administrator to run Bulb Energy when it collapsed in 2021.

A Royal Mail spokesperson said: "After 11 months of talks, making numerous improvements to our offer based on CWU feedback, and mediated talks by Acas and Sir Brendan Barber, we are deeply concerned that we have yet to reach an agreement.

"We remain committed to getting the right deal, which secures the future of Royal Mail and its workforce.

"We have been clear throughout the dispute that significant transformation of our network and working practices is essential for the business to survive.

"It is not sustainable for the business to be losing more than £1 million a day. Change cannot continue to be delayed.

"If CWU persists with further strike action, this would only serve to threaten the job security of our postmen and women and make our pay offer unaffordable."

CWU general secretary Dave Ward said Royal Mail's finances were down to the way the company has dealt with the dispute and that a proposed three-year pay deal was "not good enough."