LONDON (Reuters) -Pan-European consumers organisation BEUC filed a complaint with the European Commission on Thursday against online fast-fashion retailer Shein over its use of "dark patterns", tactics designed to make people buy more on its app and website.

Pop-ups urging customers not to leave the app or risk losing promotions, countdown timers that create time pressure to complete a purchase, and the infinite scroll on its app are among the methods Shein uses that could be considered "aggressive commercial practices", BEUC said in a report also published on Thursday.

The BEUC also detailed Shein's use of frequent notifications, with one phone receiving 12 notifications from the app in a single day.

"For fast fashion you need to have volume, you need to have mass consumption, and these dark patterns are designed to stimulate mass consumption," Agustin Reyna, director general of BEUC, said in an interview.

"For us, to be satisfactory they need to get rid of these dark patterns, but the question is whether they will have enough incentive to do so, knowing the potential impact it can have on the volume of purchases."

In a statement, Shein said: "We are already working constructively with national consumers authorities and the EU Commission to demonstrate our commitment to complying with EU laws and regulations." It added that the BEUC had not accepted its request for a meeting.

Shein and rival online discount platform Temu have surged in popularity in Europe, partly helped by apps that encourage shoppers to engage with games and stand to win discounts and free products.

The BEUC has also previously targeted Temu in a complaint.

Shein's use of gamification, drawing shoppers to use the app regularly, has helped drive its success.

In the "Puppy Keep" game on the app, users feed a virtual dog and collect points to win free items. They can gain more points by scrolling through the app, and by ordering items, but must log into the game every day or risk losing cumulative rewards.

The BEUC noted that dark patterns are widely used by mass-market clothing retailers and called on the consumer protection network to include other retailers in its investigation.

It said 25 of its member organisations in 21 countries, including France, Germany and Spain, joined in the grievance filed with the Commission and with the European consumer protection network.

Late last month, the European Commission notified Shein of practices breaching EU consumer law and warned it would face fines if it failed to address the concerns.

The company is also under scrutiny from EU tech regulators on how it complies with EU online content rules.