An Post is insisting that only a small number of customers received the Referendum Commission’s abortion information booklets with literature from the "Yes" campaign enclosed.
The revelation emerged after a number of complaints were made in Cork, Kildare and Dublin.
An Post says the mistake was caused by human error, with staff under pressure to deliver the 1.3m booklets in just six days.
Staff put the "Yes" leaflets inside the Referendum Commission booklet for convenience of delivery.
Spokesperson Anna McHugh apologised for the error but is stressed that it only affects a small number of households.
An Post says it is confident all the booklets will be delivered to households on time.