LONDON, July 17 (Reuters) - A new campaign backed by British retail consultant and television personality Mary Portas has launched an online petition calling on the new Labour government to block fast fashion retailer Shein from listing its shares in London.

China-founded Shein, known for its $5 tops and $10 dresses, confidentially filed papers with Britain's markets regulator early last month, two sources told Reuters, kicking off the process for a potential London listing later in the year.

But some senior UK lawmakers have questioned Shein's suitability and called for greater scrutiny of its labour practices, supply chain and use of an import tax exemption.

Shein has said it is committed to respecting human rights and has a zero-tolerance policy for forced labour.

Prior to Britain's July 4 election, Labour indicated its support for Shein listing in London but opposition to the move is building.

The "Say No to Shein" campaign, which is also backed by former Green Party lawmaker Caroline Lucas, wants the government to block Shein's application to list on the London Stock Exchange until it has completed a thorough investigation into its labour practices, environmental impact and tax arrangements.

Last month a UK-based human rights group Stop Uyghur Genocide also launched a legal campaign to block any London IPO.

Shein did not immediately reply to a request for comment about the new petition.

"Why would we as a country consider embracing a company like Shein onto the London Stock Exchange?," Portas said on Wednesday.

"This is a company with allegations of unethical business practices, modern slavery, and violating labour laws. Surely we are better than this?"

The petition had been signed by more than 33,000 people as of July 17. (Reporting by James Davey and Helen Reid; Editing by Kirsten Donovan)