Ikea is at the centre of a spying investigation in France over allegations that it paid private detectives to snoop on workers and pry into the private lives of disgruntled customers who complained about late kitchen deliveries or faulty wardrobe parts.

The Swedish chain, which presents itself as the friendly face of affordable furnishing, said police searched its French headquarters and the home of its head of risk management last Friday, examining computers and seizing documents as part of a preliminary judicial inquiry into alleged illegal surveillance practices.

Two French trade unions have filed legal complaints against Ikea for allegedly spying on employees by fraudulently obtaining police files. The investigative newspaper Le Canard Enchainé has published emails allegedly exchanged between Ikea's management in France and a private security firm which it claimed was charged with digging up personal information on employees and unhappy shoppers who were in litigation with the company.

The firm is alleged to have requested a range of personal data including criminal records and confidential details of any dealings with the justice system. It said scores of people had been targeted, including a union leader.

The investigative website Mediapart then detailed the cases of three customers it said were investigated by private detectives at Ikea's request after they complained about a late delivery of a kitchen and a faulty piece in a self-assembly wardrobe. Hanna and Franck F, a couple who live in the US, went to an Ikea store outside Paris in November 2006 and bought a kitchen, beds, a toilet and pieces of furniture for their holiday home in Brittany. Delivery was scheduled for December, but the final pieces were only delivered in February, they said. "In all our lives, we've never been so badly treated by a company," Hanna wrote to Ikea, complaining about the eight-week delay and demanding to be compensated for having to stay in a B&B while they waited.

Ikea compensated her, but during its dealings with the couple, the company allegedly asked a private security firm to investigate their "morality", looking into property ownership, phone details and confidential police records.

Jerome P, a 35-year-old estate agent, bought an Ikea wardrobe for his office in 2008, but found some of the wooden pieces were wonky and complained. Mediapart said senior Ikea staff had ordered a security firm to investigate him.

The daily Libération described a climate of tension among staff, quoting one trade unionist who said a new security head had installed cameras all over shops pointing chiefly at workers. One senior member of staff countered that it was because shops were often located in troubled areas where cars had been burned.

Following the first media stories on spying, Ikea France last month launched an internal investigation run by an international legal firm, saying all media reports would be taken "extremely seriously" and there would be full co-operation with the state prosecutor's investigation. Three staff-members were placed on leave of absence pending the inquiry.

"The company's ethical rules are very clear: we work with honesty and transparency, in whatever country we're present," Ikea said in a statement. "Respect for people's private lives is among the most strongly held values of the group and we strongly disapprove of any practice which calls that into question."

The firm said its own internal investigation would determine what, if anything, had taken place but that investigation was not an admission that these practices had occurred. The company would not comment further until its own inquiry was complete.