Employers were left dumb-founded after immigration officers stormed their premises causing staff to flee and arresting 15 workers.
Managers at JKW Windows in Donkey Lane, West Drayton, watched as officers from the UK Border Agency arrived in vans with blacked out windows and blocked off the exits.
Two employees were caught running from the glaziers as staff were rounded up in an early morning sting last Thursday.
Shocked manager Jaspaul Singh said: "All our paper work is in order. We've been victims because they've used forged documents. The documents look like they're supposed to look and we've followed the procedures."
Fifteen men from India, including a 16-year-old, were arrested for either working in breach of their visas or who had entered the country illegally.
They were all taken to Eaton House police station in Hounslow for questioning where they could face deportation within 72 hours.
Area director of the UK Border Agency, Gareth Redmond, said: "These arrests send a clear message to businesses in West Drayton and Uxbridge - we will not tolerate illegal working."
The company could now face a crippling six-figure fine.
Mr Singh added: "We said we were going to do a check on ID and four or five people didn't turn up for work the following day. We're going to be even more strict now."
Another manager, who didn't wish to be named, estimated the cost to the business of the raid at £15,000.
Builder Peter Paskov was caught up in the raid as he tried to buy materials. He added: "I'm really shocked. I've never seen anything like this before." [25a0] For information on employment law, visit visit www.ukba.homeoffice.gov.uk/employers or call 0845 010 6677.