TikTok “shopping hauls” are fuelling a costly surge in people returning online orders, with so-called “serial returners” sending back almost £7bn of items a year.

New figures from Retail Economics suggest that these “serial returners” account for around a quarter of all online returns made across the UK every year.

It comes amid the rise of TikTok “haul” trends, where social media users will post videos of themselves trying items on and asking their viewers which items to keep and which to return.

More than 11 million videos have been posted on the platform with the hashtag #KeepOrReturn. More than two thirds of Gen Z shoppers admit to deliberately over-ordering clothes online with a view to returning items.

High street stores are facing a glut of unwanted clothes from customers as a result. Some are frequently getting items delivered to their homes in multiple sizes and colours to see which they like, before sending back the majority of their orders.

Online returns are forecast to exceed £27bn this year. Serial returners are estimated to account for £6.6bn of the total, according to Retail Economics. On average, each “serial returner” is sending back £1,400 worth of goods each year.

Younger shoppers are more likely to be “serial returners”, according to the research from Retail Economics and returns specialist ZigZag.

A survey found that 69pc of Gen Z consumers over-ordered on sizes or colours and then returned unwanted items. This compares to just 16pc of baby boomers.

Richard Lim, chief executive of Retail Economics, said: “Serial returners are quietly eroding retail profitability in ways many retailers are only just beginning to understand.

“The rise of opportunistic shopping behaviours, where many people intentionally buy large quantities of goods with the intention of returning most of them, is placing an unprecedented strain on retailers.”

Asos early last year said frequent returners were costing it £6 per order, with some of those customers ordering two or three times a month and returning as much as 90pc of what they received.

The company is among those which has launched a crackdown on these customers.

This month, Asos introduced new returns fees for customers who were returning large numbers of orders. It said customers who frequently returned items had to pay a fee of £3.95 to send items back if they kept less than £40 worth of their order.

Others including Boohoo, H&M and Zara have also introduced charges for returns.

Mr Lim said retailers were under pressure to “urgently rethink their approach” to managing returns.