The postal service has become "chaotic" with Royal Mail workers being told to leave doctors' and hospital letters on racks to prioritise parcels, trade union bosses have warned.
Swindon is among 82 areas affected by ongoing disruption to postal services, according to the company's latest update.
Several postcodes in Swindon have been hit with disruptive Royal Mail postal delays.
Royal Mail said it aims to deliver to all addresses six days a week but admitted that factors such as sickness, absence, and resourcing issues have contributed to localised delays.
Martin Walsh, deputy general secretary of the Communication Workers Union (CWU), said: "There is a pyramid process where it is understandable that people are getting delays.
"All employees want to deliver and they know their customers, and some of them feel very aggrieved that they're told to leave doctors' letters, hospital letters in the frames to prioritise tracked.
"And we often get feedback on that issue."
Dave Ward, general secretary of the CWU, described the current situation as difficult and demoralising for staff.
Mr Ward said: "The service at the moment is chaotic.
"On a daily basis, it's extremely difficult to get through all the workload."
Royal Mail’s owner, Daniel Kretinsky, who recently gave evidence to the Business and Trade Committee, denied claims that company policy had led to parcels being prioritised over letters.
Mr Kretinsky said: "I have never heard any instruction, any discussion, or any exchange which would suggest that Royal Mail is prioritising parcels over letters.
"Categorically this is not any management decision and nobody is incentivised to do that, and we think it is actually not happening."
Mr Kretinsky, chairman of Royal Mail’s parent company EP Group, apologised to customers affected by late deliveries.
He said: "I'm deeply sorry for any letter that arrives late.
"I'm deeply sorry if we are not delivering the letters on our promise."
Royal Mail recently introduced a specialised NHS barcode to help speed up the delivery of healthcare-related mail.
However, Mr Kretinsky said that only a minority of NHS providers are currently using the new system as it is still being rolled out.
The company has also said that a delayed lorry travelling from Edinburgh to the Midlands caused knock-on delivery issues across parts of the UK, including Swindon.
