French police say they are building a case against an international gang of toy thieves specialising in stealing Lego – and they have warned specialist shops and even parents to be aware of a global trade in the bricks.

The alert comes after officers arrested three people – a woman and two men – in the process of stealing boxes of Lego from a toy shop in Yvelines, outside Paris, last June. Under questioning, the suspects, all from Poland, reportedly admitted they were part of a team specialising in stealing Lego sought by collectors.

“The Lego community isn’t just made up of children,” one investigator told Le Parisien newspaper. “There are numerous adults who play with it; there are swaps and sales on the internet. We’ve also had people complaining their homes have been broken into and Lego stolen.”

The arrested gang were first reported in France in November 2019, and again in February 2020. “They come to France, set up in a hotel in the Paris region, then set about raiding toy stores before returning to Poland to sell off their haul,” the officer said.

Gerben van IJken, a Lego specialist who advises the online auction platform for buying and selling collectibles, says sales on the French site doubled last year.

“Investing in these pieces isn’t new but this niche market has reached new heights with the pandemic. People have more time at home because of the health restrictions and the game market has exploded. We often have more than 1,000 Lego sales a week,” he said.

“There’s always been dealing in Lego because it’s a premium toy range and attracts many adults, but also because the company withdraw its collections around two years after they come out, so a secondhand market is inevitable.”

He added: “The phenomenon has exploded over the last eight years because people have realised they can make money reselling Lego on the internet.”

Van Ijken cited a Cafe Corner Lego set that cost €150 when it was released to shops in 2007 selling in its original box for €2,500 last year.

Lego looting appears to be a global business, according to reports in the US, Canada and Australia, where numerous thefts have been reported over the last five years. In 2005, San Diego police arrested a group of women found to have €200,000 worth of Lego.