The business model used by the fashion industry is broken and firms need help to adopt more sustainable practices, MPs have been told.
The warning, from a London College of Fashion academic, comes amid increased scrutiny of the UK's fashion retailers.
Prof Dilys Williams, director of the college's centre for sustainable fashion, said legislation and government support were needed.
Marks & Spencer, Primark, Boohoo and Asos have all given evidence to MPs.
Earlier this month, Prof Williams appeared before the Commons environmental audit committee's inquiry into the sustainability of the fashion industry.
In response to a question from Green MP Caroline Lucas about whether a T-shirt can be produced sustainably for £5.99, Prof Williams said: "If a business is built on fair wages and living within environmental limits then, no, we cannot sell T-shirts at the price that we currently are.
"We are buying 400% more pieces than we were less than 20 years ago... we are spending the money on stuff that we are chucking away.
"The system is broken and it cannot continue as it is. Most businesses know that, but it is about helping them to make that transition to a new form of business."
This era should be seen as a "blip in fashion history", Prof Williams said.
Phoebe English, a south London-based designer, told the hearing that High Street retailers know "their business models are just not sustainable" because young people's shopping habits are changing.
"They will not be going into Primark and coming out with five bags of clothes where garments have cost them five or six quid to purchase and buying multiple clothes - the same clothes in different colours. It is just not how people will be shopping in the future and they know their time is coming up," she said.
One way of reducing the amount of disposable fashion could be renting clothes, Ms English suggests.
Her label has been experimenting with putting pieces that cost close to £1,000 on a hiring website, which she says allows consumers to enjoy exciting designs without paying huge sums only to have clothes sit in a wardrobe after being worn once or twice.
"With hiring you get that endorphin high constantly because you can have a new thing within your purchasing power, which can help transform you as a person and make you feel better and make you look stronger and more positive, and you can do it more regularly," Ms English told MPs.
"It is a really exciting business plan that could absolutely be implicated within the High Street and it definitely should be."
Claire Bergkamp, director of innovation and sustainability at Stella McCartney, says reselling and rental are ways of disrupting the fast fashion model: "There is a generation now that has probably never seen anything outside of lower-quality product."
The members of the environmental audit committee have written to the UK's biggest fashion retailers asking how they can reduce the environmental harm.
"Three-in-five garments end in landfill or incinerators within a year ... half a million tonnes of microfibres a year enter the ocean. Doing nothing is not an option," chair Mary Creagh told the BBC in October.
Their questions to retailers include whether they pay the living wage and ensure child labour is not used in factories; whether they recycle materials and encourage recycling; how they are reducing the flow of microfibres into oceans; and whether they incinerate unsold or returned stock.
Earlier this year it emerged that Burberry destroyed unsold clothes, accessories and perfume worth £28.6m in 2017 to protect its brand and maintain its exclusivity.
Orsola de Castro, co-founder of activist group Fashion Revolution, has called burning and sending stock to landfill as fashion's "dirtiest open secret".
Burberry is unusual in disclosing the practice, however. Just six of the 35 high-end designers and High Street retailers contacted by the BBC earlier this year gave breakdowns or further information. The rest said they could not help or did not respond at all.
The fashion industry is taking steps to reduce the environmental impact of clothing, according to the British Retail Consortium. It says members are now designing products that are made to last and encouraging customers to return unwanted clothes for reuse.