Royal Mail has been criticised after Brexit leaflets, campaigning for an EU exit in June's referendum, were folded with voting guides and posted to Welsh households.
The Electoral Commission said Royal Mail was told not to fold leaflets with its material but there were instances where this had been breached.
Caerphilly MP Wayne David said it had happened "in large parts of Wales".
Royal Mail said it was not its policy to fold leaflets together.
A spokesman said: "It appears that in a small number of cases some leaflets may have been folded together with other mail simply for ease of posting through letter boxes. This is not our policy.
"All staff have again been reminded that this is not our policy. We apologise for any inconvenience caused and have already spoken to the Electoral Commission."
Mr David raised the issue in parliament, saying: "In large parts of Wales this leaflet has been distributed by post inside a very, very good leaflet from the Electoral Commission which explains the voting system for Welsh assembly and the PCC elections.
"We are trying to get to the root to how this happened but if in fact it's the Royal Mail that is responsible for this, will he [the speaker] join me in condemning them?"
An Electoral Commission spokeswoman said it was a politically neutral organisation and did not support any referendum campaigning organisation.
"We have raised these concerns with Royal Mail," she said.