Katrina Harradine, 29, purchased the two-piece Stripe Tie Triangle High Waist Bikini for an upcoming holiday in St. Lucia.
She usually buys a size 16 and purchased the bikini along with several other items including a swimsuit, shorts and a top in a $6 deal online.
Harradine then stockpiled the summertime wear and left it in the packaging ready for her holiday on June 23 to see family and friends.
However, when Harradine tried on the bikini, she was left stunned after discovering Boohoo had incorrectly sent two size eight bikini bottoms.
To make matters worse, she claims the proper size 16 bikini top is made to fit a "16-month-old baby or a woman with just one breast".
The freelance project manager took to Twitter to call out the UK retailer and shared photos of the ridiculous gaffe.
She wrote, “Hey @boohoo_cshelp did y’all run out of size 16 bikini bottoms so send me two size 8’s as a maths joke?? I actually wish I was kidding!”
Harradine said to SWNS, “Sizing is never perfect but you don’t ever expect anything as bad as this.
“You usually end up ordering and hoping for the best but this is ridiculous.”
Harradine, who lives in South East London, spotted a $6 deal at Boohoo and decided to purchase several bikinis and outerwear in preparation for a seven-week annual holiday to the Caribbean.
She placed the order on May 22 and it arrived a few days later but Harradine decided to store all the items and try them all on at a time nearer to her holiday.
But following a chance conversation with her cousin about a candy-themed party whilst she is away in St. Lucia, Harradine opened the Boohoo order and to her horror discovered the bizarre mix-up.
Harradine claims she has been given a "feeble" apology and told to send the package back along with a note.
“I ordered a whole bunch of stuff on a $6 deal," she added. "I’m going to the Caribbean on June 23 for seven weeks and needed some new bikinis.
“I ordered about six things, it had everything from bikinis to summertime outerwear. The package came and I just put it in the corner with the rest of the items I’d ordered.
“I had been buying things and then stockpiling them ready for the holiday. I thought I would try everything on in one go.
“But I was speaking to my cousin who is a fashion designer and I told her there is going to be a candy-themed party in St. Lucia.
"She was coming up with ideas of what I could wear and I said I'd bought a striped bikini which could work.
“I couldn't believe it when I opened the package. I thought to myself, 'why me? Why do I always get these ridiculous things happening to me'?
"I decided to post it on Twitter because I thought I could have some fun with it.
“I’m lucky I had a conversation with my cousin. I opened it up and put it against myself and go only fit one leg through.”
Boohoo has been contacted for comment.