Police have arrested 23 employees of online DIY retailer Screwfix on suspicion of illegal working.
The arrests come eight months after a staff shortage forced the company to stop taking orders for a brief time.
Richard Butler, HR director at Screwfix, said it was too early to say if the staff concerned had been working illegally.
The company had "robust recruitment criteria" to ensure that all staff were genuinely entitled to work in the UK, he said.
Butler told the BBC: "It would now appear that this process may have been breached, in certain cases, during our intensive recruitment drive in the latter part of 2004."
Police, immigration officers and officials from the Department for Work and Pensions staged the joint raid just before 5am at Screwfix's distribution centre in Stoke.
They spoke to 200 workers and arrested 25 of them, two of them in connection with unrelated matters.
The others are being questioned at local police stations about suspected illegal working and fraudulent use of National Insurance numbers.
The Home Office said Screwfix co-operated fully with the operation.