A woman has criticised Royal Mail for failing to deliver important health documents for her husband on three separate occasions.
Angela Haywood, from Kent, said: "I have found it very very stressful, I have woken up in the night worrying about it."
She has now resorted to delivering most of her Christmas cards by hand as she has lost faith in the service.
Royal Mail said its teams were working hard to ensure a reliable service.
Mrs Haywood paid £25.99 for next day delivery and the documents did not arrive on time.
She said: "I can't be the only person in this position. What is going on?"
BBC South East has been contacted by others affected by postal delays in the region, which have led to missed hospital appointments.
Lewes MP Maria Caulfield said the level of service was "unacceptable" at the moment.
She has called for a meeting with Royal Mail after she said she had been "inundated" with messages from her constituents.
Crawley MP Henry Smith said that the Post Office and Royal Mail, which was sold off in 2015, should be brought back into public ownership.
One postal worker told the BBC that staff have "no pride to work there anymore" after huge changes to their contracts following privatisation.
They said that there was a "staff shortage" and that Royal Mail was a dreadful company to work for and the customers had "no benefit".
They added: "Parcels do get priority delivery as they take up more space in the sorting offices".
A spokesperson for Royal Mail said: "Our posties are an integral part of Christmas, connecting loved ones and we are proud of the work they do.
"Since the summer, we have recruited more than 7,000 postmen and women, and in the lead up to Christmas we have held a recruitment campaign hiring up to 16,000 seasonal workers."