Lawmakers in the U.K. have questioned online "fast fashion" retailers about the impact of their production processes on workers and the environment.

Mary Creagh, chair of the U.K. government's Environmental Audit Committee, wrote to five online-only fashion retailers — including global firms Amazon and ASOS — to request information on areas including staff wages, the life-cycle of the garments sold, and steps being taken to reduce the environmental and social impact of their businesses.

The committee said it was considering policy recommendations on reducing the harm caused by cheap garment production, and would factor in the retailers' responses when delivering those recommendations to fellow lawmakers.

Published Friday, letters to Amazon and ASOS asked the retailers how they had ensured that all garment workers are paid the minimum wage; whether they educated suppliers on the cost of U.K. labour; and what recycled materials were used in their products.

Retailers were given a deadline of November 15 to respond to the letters, with MPs suggesting they could ask certain companies to give evidence in parliament.

In a statement on Friday, Creagh said evidence the committee had heard on October 30 from industry experts "raised alarm bells about the fast growing online-only retail sector."

"Low quality £5 ($6.51) dresses aimed at young people are said to be made by workers on illegally low wages and are discarded almost instantly, causing mountains of non-recycled waste to pile up," she said.

"We will be calling some of these online retailers in front of the Committee to answer questions, but in the meantime, my letters encourage them to face up to the social and environmental consequences of their business models. We want to know that they are fully compliant with employment law, that garments have a decent life-span, and that profit is not put before environmental damage."

Stella Claxton, senior lecturer in fashion management, marketing and communication at Nottingham Trent University, said the UK fashion industry in its current form was "not that environmentally sustainable."

"Coupled with that we have a situation where chasing low prices has led to global supply chains looking for cheaper manufacturing, which is normally in developing countries," she said. "Thinking about how consumption in Asia is going to rise in the next few years and how UK brands are looking to service those markets. Although it is a UK problem, it is a global problem as well."

Further evidence raised concern that suppliers in developing countries outsourced to sweatshops or were not paying workers the minimum wage. "Sweatshop" is a term for a workplace that has very poor, socially unacceptable working conditions.

Amazon declined to comment on the issue. In an email to CNBC, an ASOS spokesperson said: "ASOS is looking forward to co-operating with the committee."

WATCH: Here's how Target is making a comeback