Retailers struggling to cope with a bigger than expected online shopping boom are taking too long to deliver items, resulting in a surge of complaints from shoppers. Research by retail analysts at accounting firm Accenture show retail delivery times during the Christmas period have gone up, with firms now taking an average of five days to deliver, compared to 3.6 days last year.It tracked the deliveries of 57 retailers and found one in three are slower at getting parcels to their owners than their stated maximum delivery. A quarter of retailers admitted they were struggling to cope with purchasing demands at peak times.–– ADVERTISEMENT ––And in a further kick in the teeth, customers are also being asked to spend more in order to qualify for free delivery, with the spending threshold for free delivery rising by 5 per cent compared to last year, it found. It comes after official stats showed retail sales jumped more than expected in November, helped by Black Friday promotions and stronger growth in sales of household goods.Official figures showed sales rose 1.4 per cent from October, despite economists’ forecasts of a 0.3 per cent gain.Separate data from complaints handling firm Resolver shows complaints about online shops and delivery firms in December has soared when compared to the same period last year. This December it has taken 443 complaints a day about online shops and delivery firms, having tripled from last December when 166 complaints a day were taken. Siobhan Gehin, managing director at Kurt Salmon, part of Accenture Strategy, said: “While many retailers delivered an excellent customer experience, a surprising number of retailers struggled to deliver a strong ordering and delivery experience across channels. Those retailers now only have a limited window to fix problems with longer delivery times and technical issues ahead of the Boxing day and January sales, if they’re to satisfy customer’s high expectations.“Certain UK names are consistently strong while others need to re-focus investment to enhance their competitive agility and ensure they’re equipped to deal with promotional peaks. As brands face fierce competition and wavering customer loyalty, poor performance during these crucial calendar events can leave a lasting impression. “ Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings.