An MP has claimed that Royal Mail staff were told to empty a busy sorting office before she visited to give the false impression that it was not dealing with backlogs.
Melanie Onn went to the office in her Great Grimsby and Cleethorpes constituency following complaints of delivery delays.
The Labour MP said when she arrived mail frames and portable cages were empty and "all operations seemed to be going exceptionally smoothly without any problems whatsoever".
But she has now written to the small business minister after being told by post workers that managers had issued instructions to clear the office "to demonstrate that there were no problems, nothing to see here".
Royal Mail said the claims were "simply not true".
Ms Onn said she visited the office on 1 May after raising concerns about disruption caused by large volumes of parcels and a "substantial backlog" of mail.
In her letter to small business minister Blair McDougall, she wrote: "I had also heard that some delivery rounds have gone undelivered for up to nine days, with post left in the office day after day.
"There was frustration that parcels were being prioritised despite staff reportedly being advised that this should not be the case, leaving letters and important documents undelivered."
When she visited, she said, she was given a "different impression" and felt her visit was "entirely unnecessary".
But she added: "Since then, I happen to have had conversations with a number of post workers while they've been out on their rounds.
"They each independently disclosed that management instructions on the day of my visit told them to clear the frames of post and take it out of the office. Some of the mail remained unsorted in the boxes.
"The intention was to show a clear office, and demonstrate that there were no problems, nothing to see here.
"However, every single one of the postmen I spoke to all independently corroborated that the mail removed that day, reappeared at their frames on the Tuesday morning following the bank holiday weekend.
"This meant they then had four days' worth of mail waiting for them that morning."
Ms Onn has called for Royal Mail to be honest about its challenges.
Royal Mail denied the claims.
A spokesperson said: “These claims are simply not true. We invite MPs into our delivery offices so they can see the operation for themselves.
"Mail goes out for delivery in the morning, so offices are naturally quieter in the afternoon. That is not evidence that mail has been removed or hidden.
“We closely track service performance across our network, which is how we know more than 92% of letters are delivered on time. We also carry out around 100 unannounced spot checks every week to check mail is being managed properly.”