The UK Borders Agency said six people were arrested during a raid at Euro Car Parts' headquarters in Wembley.
The arrests were for matters such as invalid visas, fake documents, or having entered the UK illegally.
Euro Car Parts was served with a civil penalty following the investigation.
Potential £10,000 fine
ECP has until the end of September to prove that it had carried out appropriate checks on workers, and a further 28 days to appeal any decision made by UKBA.
If it fails to show that pre-employment checks were in place, it could be fined up to £10,000 per illegal worker.
The UKBA said the onus was on companies to make sure that staff had all the correct documentation to work in the UK.
Changes in law on immigrant workers
The Home Office is currently replacing the work permit system with a points-based system for immigrants.
In addition, employers will need to obtain a licence to employ people from outside Europe.