Aimee Smale, 23, from Grays, Essex, set up her own label, called Odd Muse.
The designer launched her favoured sand-coloured Ultimate Muse blazer.
But a near identical one appeared on the PrettyLittleThing website weeks later.
A young fashion designer has lambasted British clothing firm PrettyLittleThing for 'stealing' her £135 blazer look and creating a near-identical version for just £30.
Aimee Smale, 23, from Grays, Essex, quit a two-year career in the buying office at ASOS in August to set up her own label, which she called Odd Muse.
But within a week of her launching her favoured sand-coloured Ultimate Muse blazer, a very similar one appeared on the PrettyLittleThing (PLT) website.
She says she is convinced staff at the Manchester-based retailer saw the unusual jacket as several PLT buyers had followed her business Instagram account not long before the item appeared on the site.
Fortunately, when she tweeted about her upset, it resulted in her business shifting a further 700 extra tops - and she now expects to sell hundreds more as the waiting list for the product is 'huge'.
Aimee, who runs her business from her kitchen table, said: 'It's infuriating. My business has been about a year in the making. I created the brand as a response to the fast fashion industry.
'The whole brand is introducing a slower side to fashion to young women my age, who are shopping labels like PLT and Boohoo.
'I'm encouraging the customer to spend more on their clothes and make it more of an investment, than constantly spending on this "wear it once and throw it away" culture.
'Girls my age are tempted by the ridiculously cheap prices, but it's unethical in every way. So when they copied my design I felt like it summed them up.'
Aimee originally launched her range online in early September, with a six-piece collection - ordering 50 units of each product, except the £135 blazer, which she predicted would be popular so gathered 100.
But she was heartbroken to see an 'identical design' on PLT's website just over a week later in beige and red for a knock down price of £30.
Aimee says she has complained to the retailer, but is yet to receive a reply. PLT has been contacted for comment.
The designer posted of her upset on Twitter, which prompted an incredible 8,000 likes and almost 5,000 retweets, with shoppers flocking to offer their sympathy.
She tweeted: 'As a small business and independent designer, I want to be heard. The Fast Fashion Industry replicates years of hard work in a matter of minutes & it is NOT fair. Please read and share.'
She also claims fellow retailer Miss Pap's collection with influencer Tia Lineker used her branding, calling the capsule The Ultimate Muse Collection a day after she released her Ultimate Muse Blazer.
After her post went viral, Aimee claims PLT even had the gall to reply to customers tweeting her asking when her blazer would be restocked to tell them they too sold it.
'Loads of people have posted it across their social media and it has just gone crazy, so in a way I guess it's been quite good for me,' Aimee went on. 'But it was annoying to see. I spent a year designing this jacket and PLT did it in a week.'
She ordered the PLT jacket herself to compare her product with theirs and said the quality was world's apart.
'They can't really match the shape,' Aimee said. 'My blazer is really thick with the stitching and the attention to detail and Pretty Little Thing's is paper thin. The belt is exactly the same, but it's very flimsy.
'I go through so much to pick the perfect, best quality fabric. And I have that over them. Anyone who is shopping my brand is really not about producing things as cheap and as fast as possible.
'I always tell my customers, you can pass my clothes on to your daughters. And I think that's so lovely.'
Supporters on Twitter said they were upset to see the similar design, with one writing: 'I'm so sorry this happened to you! I despise fast fashion for this exact reason!
'People are slowly starting to realise the detrimental effects of fast fashion and are becoming more conscious shoppers so it won't last them long!'
Another said: 'This is so awful, bless you - best of luck with your business though it looks amazing!'