Postal deliveries in a part of south London are still being affected by last month's postal strikes.
The strike was over pay and conditions
Mail delivery watchdog Postwatch said it had received a number of complaints about deliveries in Streatham, south London, since the official strikes.
It said some residents and businesses were getting late deliveries, and on some days no delivery at all.
The Royal Mail said unofficial strikes and "poor industrial relations" were to blame for the backlog in Streatham.
The October strikes were the results of a bitter dispute over flexible working conditions. The Communication Workers Union (CWU) has since endorsed a Royal Mail offer.
The series of 48-hour stoppages and unofficial walkouts was estimated to have cost Royal Mail more than £200m.
"Royal Mail need to resolve the operational and industrial relations problems behind these services failures," said Terry Stafford of Postwatch.
"In the meantime we are asking that they keep customers informed about the extent of the problem, so that people can make alternative arrangements where possible."
A Royal Mail spokesman said: "We apologise for the delay in clearing remaining backlogs in the Streatham area.
"This is due to the unofficial industrial action in parts of south and east London which followed the CWU industrial action and is also due in part to strained industrial relations.
"Customers can be assured we are doing all we can to clear the remaining few backlogs as soon as possible and this has included drafting in extra management volunteers and reserve staff."