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Most of the clothes contain synthetic fabrics like nylon, acrylic and elastane, which are made with fossil fuels and damage the environment through emissions and waste, the Royal Society of Arts, Manufactures and Commerce (RSA) said.
"These fabrics ... form part of a petrochemical economy which is fuelling runaway climate change and pollution," said Josie Warden, the RSA's head of regenerative design and co-author of its report "Fast Fashion's Plastic Problem".
Britain's throwaway culture means most fast fashion will end up in landfill where it could take thousands of years to break down, said the RSA, which works to find solutions to social challenges.
Published ahead of the start of London Fashion Week on Saturday, the report said fast fashion companies were too slow to adopt recycled materials.
Only 1% of clothing on PrettyLittleThing's website contained recycled materials, 2% on Boohoo, 4% on ASOS and 5% on Missguided, according to RSA's analysis of 10,000 items recently listed by the brands.
It accused fast fashion companies of "greenwashing" their images by producing small sustainable ranges, while most of their products were made from petrochemicals, whose use must be curtailed to combat climate change.
ASOS said it was not a fast fashion brand, designed clothes to last and educated customers on prolonging the life of garments. Other companies did not immediately respond to requests for comment.
The study found 89% of merchandise on PrettyLittleThing's site contained new plastics, 84% on Boohoo and Missguided, and 65% on ASOS.
Overall, 49% of garments were entirely made of new plastics, rising to 60% for Boohoo.
With the use of synthetic fibres in fashion doubling between 2000 and 2020, the report said Britain - which is hosting the global COP26 climate summit in November - must take action to create a more sustainable fashion system.
"The sheer volume of clothing produced by these websites is shocking - we should see many of these items, which go for rock-bottom prices, as akin to other short-lived plastics," Warden said.
"The nature of fast fashion trends means they are not designed to have long lives in our wardrobes."
The RSA said income from a tax on clothing containing virgin plastics could be invested in creating new materials, recycling and boosting more sustainable production.
The authors said most shoppers were unaware of the scale of plastic use in fast fashion.
They called for brands to publish regular statistics on how much plastic goes into their garments and explore ways of promoting second-hand clothing.
- Thomson Reuters Foundation