Christmas shoppers have been urged to order online gifts by Saturday December 12 to avoid “delay dismay”.
Experts warn leaving it any later risks items ordered using standard delivery not arriving before Christmas Day.
Royal Mail acknowledged delays due to “exceptionally high volumes”, with shop closures ramping up pressure.
The surge in online shopping during the pandemic has also placed a strain on couriers firms.
Industry giant Hermes said: “We are seeing unprecedented volumes – around three million parcels each day – the level we expected for 2025.”
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Rival DPD said parcel volumes were up 40 to 50% on last Christmas.
Price comparison website Parcelhero warned of a delivery “meltdown”.
Some retailers have brought forward cut-off dates for Christmas deliveries.
Boots warned of potential issues, but Very promised gifts ordered up to 7pm on December 21 would arrive on time.
Meanwhile, £2.5billion is predicted to be spent on the high street this weekend.