Online retailers Shein, Temu, AliExpress and Wish are being investigated in France in relation to the offence of enabling minors to access pornographic content on their platforms, the Paris prosecutor said on Tuesday.

The country's consumer watchdog had reported the four firms to the prosecution service on Sunday after raising concerns about the sale of childlike sex dolls on Shein's platform over the weekend.

The Paris prosecutor's office told the BBC that the platforms are being investigated over violent, pornographic or "undignified messages" that can be accessed by minors.

AliExpress told the BBC it takes the matter very seriously. Shein, Temu and Wish have also been contacted for comment.

Shein and AliExpress are also under investigation over the dissemination of content related to children that are of a pornographic nature, the office said.

The cases have been referred to Paris' Office des Mineurs, the prosecution service added. The office is an arm of the French police force that oversees the protection of minors.

AliExpress said the listings in question violated its policies and were removed once it became aware of them.

"Sellers found to violate or trying to circumvent these requirements will be penalised in accordance with our rules," AliExpress said in a statement.

On Monday, Shein said it had banned the sale of all sex dolls on its platform worldwide. The Singapore-based retailer also said that it would permanently block all seller accounts related to the illegal sale of the childlike dolls and set stricter controls on its platform.

The French consumer watchdog, the Directorate General for Competition, Consumer Affairs and Fraud Control, had said the sex dolls' description and categorisation left "little doubt as to the child pornography nature" of the products.

The scrutiny of Shein comes as the company, which was founded in China, prepares for the opening on Wednesday of its first permanent physical outlet in France .

Protesters have been seen gathered in front of the Paris department store where Shein is set to open the outlet.

Shein plans to open outlets in other French department stores in cities including Dijon, Reims and Angers.