Glasgow designers Timorous Beasties has accused Topshop of ripping off a design for its onesie range.

The design firm, formed by Paul Simmons and Alistair McAuley in 1990, says the retail giant has lifted one of its most famous patterns.

The Topshop design features similar elements to those that appear in Timorous Beasties's distinctive London Toile wallpaper design.

The designers posted pictures online of their London Toile wallpaper pattern next to an image of the Topshop onesie with the headline "Spot the Difference – Real or Rip-Off?" The Facebook posting describes the retail store's garment as a "Topshop rip-off of our London Toile design".

It adds: "They have actually used our artwork. Some parts have been redrawn but for the most part it is our exact drawings. They have even lifted drawings from our New York Toile also and incorporated them. Shame on them."

A spokesman for Topshop said: "At this time we have not been contacted by Timorous Beasties with regards to the allegation, the nature of which we take very seriously. We are currently investigating this matter."

Last year a young designer and illustrator, Kate Moross, attacked Topman – a company, like Topshop, owned by the Arcadia group – on Twitter for copying a print featured on one of her sweatshirts.

She tweeted: "Hey Topman look forward to hearing from my lawyers. Ripping off independent designers has to stop." The item in question was withdrawn from Topman's online store shortly afterwards.

A month earlier the company settled a dispute with Fred Perry over claims it used a label on a jumper similar to Fred Perry's logo.