Import duties kick in for shipments worth £135 or more.
While HM Revenue & Customs said it did not have figures for how many parcels fall under this threshold, other countries provide some clues as to the scale of deliveries.
In the EU, the number of parcels under the bloc’s customs threshold – a declared value of less than €150 (£126.89) – hit two billion last year.
In the US, official figures show that the number of shipments that avoided customs charges jumped 67pc between 2018 and 2022 to 685.5m.
A House of Representatives committee said it believed Shein and Temu, another Chinese retailer that ships directly from factories, accounted for around a third of these.
Officials have been plotting various steps to close tax loopholes in the US and EU.
Richard Allen, from Retailers Against VAT Abuse Schemes, says Britain has been slow to act and blames Brexit, which he argues left gaps in the system for collecting VAT and duties.
Allan says there is an opportunity for the next government to “come up with a solution [to the tax loopholes] that would be better than everyone else[’s] in the world”, given the Brexit freedoms.
So far, both Labour and the Conservatives have shied away.
Warnings from high-street retailers have gone unanswered.
Time is not on their side.
“Shein is going to expand into multiple categories beyond fast fashion,” says Brittain Ladd, a former Amazon executive.
“This will result in a massive increase in the amount of products Shein imports. Shein will decimate retailers in the UK.”
Labour may be keen to land London a blockbuster stock market float as part of a drive to bolster growth.
But retail bosses are anxious this does not come at the expense of UK businesses.
“It would be like shooting themselves in the foot,” one chief says.