The Communication Workers Union (CWU) has told Royal Mail it must make significant progress in resolving its pensions dispute by 6 September to avoid industrial action.
The "pragmatic" deadline, which has been extended from 15 August, has been imposed to enable the union and Royal Mail to have more time to discuss and reach an agreement on future pension provision.
The postal firm is battling with members to close its defined benefit (DB) scheme to future accrual next April and replace it with a "cash balance" scheme. The arrangement would see members receive a guaranteed lump-sum payment at retirement, based on total contributions of 19.6%, which would also increase depending on investment performance.
Royal Mail said the move is needed as it estimates the current £3.8bn surplus will be swallowed up by increased running costs, leading to contributions surging from 17% of pensionable pay to 50%.
Speaking to members in a video on Facebook, deputy general secretary for postal Terry Pullinger said the new timetable was responsible.
"We have designed a timetable to give Royal Mail Group a further chance to move things forward sufficiently so we don't have to give notice for industrial action," he said. "That, of course, is our key objective: to reach an agreement here.
"That builds in a further two weeks for talks to take place should the company seriously wish to make progress. I believe that is a pragmatic solution. I believe that is the trade union being responsible.
"There is every opportunity now for the company to come into the room and negotiate a settlement that is one that we would consider sufficient progress to avoid giving notice for industrial action."
The company's proposal is in response to a risk-sharing scheme suggested by the CWU, where the DB and defined contribution (DC) schemes would be merged, with all 132,000 members receiving a guaranteed wage in retirement. This could then be uplifted in line with inflation depending on investment performance.
A Royal Mail spokesperson said it was "committed" to finding a resolution with the CWU but there are "no grounds" for industrial action.
"Any potential ballot in the future does not mean there will be industrial action. Industrial action - or the threat of it - undermines the trust of our customers," they said. "We are committed to providing our customers with the high quality service they expect from Royal Mail.
"Under its proposals as part of the ongoing negotiations, Royal Mail would continue to provide the best pay and terms and conditions in the industry. There would be no change to our core terms and conditions, such as sick pay, holiday pay and our commitment to permanent employment for the vast majority of the workforce."
Pullinger also called on more Royal Mail senior managers to come to the negotiating table, stating an "agreement would be more forthcoming [and] the negotiations would be more immediate in terms of finding a solution".
The Royal Mail spokesperson said senior individuals were involved when appropriate: "A range of negotiations are underway with the CWU involving many senior Royal Mail executives. As in previous negotiations, these are being led by senior executives from our operations business and our industrial relations professionals.
"As the negotiations progress - as on previous occasions - other senior executives at the group level participate in specific discussions. That will continue to be our approach."
The warning comes after a Unite ballot revealed members only narrowly rejected the proposals, prompting the union to call for greater clarity and information from Royal Mail.
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