(Reuters) -UK-based human rights group Stop Uyghur Genocide has launched a legal campaign to block Shein's potential London listing over concerns about its labour practices, a law firm representing the campaign group said on Wednesday.

Human rights law firm Leigh Day has written to the UK's Financial Conduct Authority (FCA) to urge the regulator to refuse any attempt by Shein to list on the London Stock Exchange (LSE), it said.

"SHEIN has a zero-tolerance policy for forced labour and we are committed to respecting human rights. We take visibility across our entire supply chain seriously and we require our contract manufacturers to only source cotton from approved regions," Shein said in a statement.

The FCA declined to comment.

On Tuesday, Amnesty International UK said Shein's potential London initial public offering would be a "badge of shame" for the LSE because of the fast-fashion firm's "questionable" labour and human rights standards.

Shein confidentially filed papers with Britain's markets regulator in June, two sources told Reuters on Monday, kicking off the process for a potential London listing later this year.

The company has previously said it was investing in strengthening governance and compliance across its supply chain.

The FCA does not have investigation or enforcement powers related to alleged breaches of legislation not within its remit, such as the Modern Slavery Act or tax legislation.