Evri, the courier company formerly known as Hermes, has apologised after a rising number of complaints about parcels not arriving.
Clothing retailers ASOS, H&M and JD Sports have received hundreds of queries about missing deliveries, the BBC reported today.
Complaints include packages stuck at depots, delays with no follow-up information and paid-for next-day deliveries not arriving.
Evri, which said it is delivering 3m packages a day currently, told the BBC it was "sorry customers in certain local areas are experiencing short delays in receiving their parcels".
Customers switching to Evri due to strikes at Royal Mail had caused problems it said, while it is being affected by staff shortages in some areas.
JD Sports confirmed some carriers had seen "higher than expected volumes" as a result of Royal Mail strikes, while ASOS told the BBC it was advising customers to allow an extra 72 hours for delivery.
German-owned Evri was recently voted as worst for service for a second year running by Citizens Advice.
It rated an average 1.75 out of 5 in the 2022 parcels league table compiled by the consumer advice service with IDS-owned Royal Mail being the best of the bunch with a score of three.