The CWU plans further industrial action all next week in response to the continuing deadlock in talks with Royal Mail over pay, pensions, jobs and working conditions that has caused serious disruption to the UK postal industry since June.
The union said "real progress" had been made in a number of key areas with Royal Mail, but that there had been no agreement reached to avert further strikes, which have involved walkouts by 130,000 union members.
The latest round of strikes, which have crippled the UK postal service since late last week, have been condemned by the Prime Minister, who said yesterday: "This is unacceptable, this is disruption to people's lives... I want these people back to work."
In a statement, the CWU said Royal Mail had improved its initial 2.5% pay increase offer to 6.9% over two years, but that it was subject to "unacceptable strings" involving a reduction in pension benefits, and no guarantees on set working hours, which are believed to be the final issue of contention among workers.
The CWU has also taken out full-page ads today in newspapers including The Times, asking the public to write to their local MP about Royal Mail's deal over pay, pensions and working conditions.
On Thursday, 12,000 Royal Mail managerial staff reached a deal with Unite to avert further strikes, involving a 2.5% rise in base salaries, and backdated promotion increases and allowances from April 1. It is still hoped that the breakthrough in negotiations will have a knock-on effect on frontline Royal Mail workers.
Adam Crozier, chief executive of Royal Mail, has dismissed as "cobblers" claims made by Dave Ward, deputy general secretary of CWU, that its present working hours were tantamount to slavery.
He told BBC Radio 4's 'Today': "I doubt there is a company out there that does not ask its people to work to all the hours they are paid for. For the union not to accept that is not a tenable position.
"All we are asking is that people work the 37 hours 20 minutes for which they are paid. If they work longer than that, of course they will receive overtime."
Meanwhile, the CWU has responded to claims made by Royal Mail that the strike action had not received full backing from union members.
Billy Hayes, general secretary at the CWU, said: "Royal Mail's claims regarding the numbers of people at work are a poor attempt to detract from the truth that postal workers are rejecting their proposals in overwhelming numbers.
"They should stop using their efforts to spin and start putting them into reaching an agreement."