Royal Mail has been accused by a senior MP of breaching its legal obligations by prioritising parcels over letters.
Business Committee chair Darren Jones said a poster in a delivery office sent to him by a whistle-blower had told workers to favour delivering parcels.
He showed it to Royal Mail chief executive Simon Thompson who was giving evidence to the committee.
Mr Thompson said he was aware of the sign but that it was "absolutely not our policy".
Showing Mr Thompson the poster, Mr Jones said "you are unilaterally only delivering on 50% of the USO (Universal Service Obligation)" and asked him if the poster represented Royal Mail policy.
Under the Universal Service Obligation, Royal Mail is legally required to deliver letters to every address in the UK, six days a week, at a uniform price, and parcels five days a week.
Mr Thompson denied that the message on the poster was company policy, adding that the poster was a "local action" and that it had been "dealt with".
But the committee chair sounded unconvinced by what he heard. "I'd remind you Mr Thompson that misleading Parliament is not something that we appreciate here on the committee.'' And he asked the chief executive to write to the committee to ''prove'' that this was not Royal Mail policy.
David Bharrat, a postal worker who is also a branch secretary for the Communication Workers Union, says the poster was displayed in a delivery office in the Harrow and District area.
He said the sign appeared in August last year and told the BBC: "We feel depressed. We're leaving mail behind and that's not what we're about, postmen regard all mail as important."
Mr Bharrat said that letters at his delivery office regularly sat in sorting devices known as "frames"' for four days at a time.
He said that the poster would not have been put up on the wall without the permission of a senior manager. "Local managers do not do anything without getting advice from senior management."
Earlier in the hearing, the Conservative MP for Rushcliffe, Ruth Edwards, said that postal workers from across the country had contacted her to say that they had specifically been told to prioritise parcels.
Mr Thompson said that letters and parcels were given equal importance by the company. "We've been very, very clear that there is no difference between the two. We've written to our teams on a regular basis, I remember in my first year I wrote to every postal worker reminding them of the importance of letters."