DUBAI/HONG KONG (Reuters) - Online fast-fashion retailer Shein is set to cut its valuation in a potential London listing to around $50 billion, said three people with knowledge of the matter, nearly a quarter less than the company's 2023 fundraising value amid growing headwinds.

The company's business prospects have come under a cloud in recent days after the Trump administration said it would close the "de minimis" duty exemption in the United States, ending an import rule that had helped Shein keep prices low.

The measure's removal could hurt Shein's profitability and push up product prices in the U.S., its biggest market, analysts and industry experts have said.

The eventual IPO (initial public offering) valuation will depend on the impact of the end of de minimis on the retailer's business, one of the people said. Given the removal only took place this week, it will take time to assess, they added.

Shein and rival Temu together probably accounted for more than 30% of all packages shipped to the U.S. each day under the de minimis provision, the U.S. congressional committee on China said in a 2023 report. The measure exempted shipments of less than $800 from import duties.

The sources declined to be named as they were not authorised to speak to the media.

Shein, founded by China-born entrepreneur Sky Xu, did not respond to a request for comment.

The removal of de minimis is part of President Donald Trump's imposition of an additional 10% tariff on China in what he called an "opening salvo" in a clash between the world's two largest economies.

Nearly half of all packages shipped under de minimis come from China, according to the congressional committee report.

Shein had been aiming to go public in London in the first half of this year assuming it secured approvals from regulators in the U.K. as well as in China, Reuters reported last month.

The company's last fundraising round in 2023 valued it at $66 billion, about a third less than its peak a year earlier, sources have told Reuters.

The latest target IPO valuation would mark the retailer's second consecutive down round, when a company takes a hair cut on its valuation during a funding round.

The reasons were not immediately known.

Shein's proposed IPO comes at a time when the UK government has been pressuring its regulators to be more pro-growth and has launched an overhaul of listing rules to make London a more attractive market to companies.

A UK government source who declined to be named as they were not authorised to speak about the deal publicly said it was still keen for Shein to launch an IPO in London.

Shein confidentially filed papers with Britain's Financial Conduct Authority (FCA) in early June, sources told Reuters last year. However, it has taken longer than typically expected for the regulator to sign off on the listing.

The FCA has not made any decision to approve the IPO yet, a separate person said. The FCA declined to comment.

Market experts say it usually takes several months to reach a decision. A spokesperson for the FCA previously said timelines for IPO approval depend on each individual case.

Shein shifted its plans to list in Britain last year, ending an attempt at a U.S. IPO after pushback from lawmakers concerned about alleged labour malpractices and lawsuits from competitors.

The IPO will also need approvals from Chinese regulators notably the China Securities Regulatory Commission (CSRC), sources have told Reuters.