Around 120 staff at the Manvers delivery office in Wath, which deals with post deliveries throughout the Dearne, are taking industrial action following a revision of shifts introduced last year.
Workers are upset because Royal Mail bosses have taken an hour and a half's work time off the Tuesday and Saturday shifts and put it onto Thursday and Friday.
Staff now claim the extended working days are too long and there is not enough time to meet deliveries sufficiently on the shorter days.
It is believed Royal Mail bosses also claim work done on overnight shifts can be done during the day so these night shifts have been scrapped as part of the restructure.
Gary Hodkinson, North East representative for the Communication Workers Union (CWU), said: "We all want a more efficient service for the public but we don't think these changes will improve things.
"The shorter days mean staff will struggle to meet deliveries and then they will be lumped with very long working hours later in the week.
"Ideally we would want the hours put back to how they were."
Since the changes were implemented last October workers have taken strike action on 17 days, including Saturday and this Monday, after talks between the CWU and Royal Mail bosses broke down.
While some staff were picketing outside the Manvers delivery office this week, about 40 postal workers handed out leaflets to the public and marched through Mexborough High Street while holding aloft placards and banners.
Further strike action is planned for two days a week over the next fortnight and workers are looking at carrying out more rallies and marches in Goldthorpe and Wath town centres.
Mr Hodkinson said: "We marched through Mexborough and handed out leaflets because the office in Manvers is on the middle of an industrial estate so it is hard to let the public know what our situation is."
A Royal Mail spokesman said: "Royal Mail is disappointed that the union is planning further strike action. We continue to do everything possible to resolve the situation and to minimise disruption.
"While our contingency plans enable us to deliver as much mail as possible, including Special Delivery items and firms' mail, we
apologise to customers whose mail may be delayed.
"The changes in work practices at the centre of this dispute were implemented following agreement with Communication Workers Union (CWU) area representatives, but staff at Manvers Delivery Office, which covers postcode areas S62, S63 and S64, have rejected them."
Union leaders and Royal Mail bosses were due to meet in London yesterday afternoon, after the Times went to press, in the hope of reaching an agreement.
lee.peace@dearnetoday.co.uk