When RTI starts in April 2013 if you employ foreign staff you may have to submit their passport number to the revenue along with all their other details.

Here is an extract from the HMRC website

Avoiding employing someone illegally

If you're going to employ someone, you need to make sure that they're entitled to work in the UK before they start working for you. Just because they can give you a National Insurance number doesn't prove that they're entitled to work in the UK.

The UK Border Agency recommends that employers conduct checks on entitlement to work for all new employees to prevent illegal migrant working. If you don't complete these checks, you could be charged a civil penalty of up to £10,000 per illegal worker. If as part of these checks you have reviewed the employee's passport, you should make a note of the passport number because you need to send this to HMRC in the employee's first Full Payment Submission (FPS) to report their payroll information. http://www.thewagesroom.co.uk/news.php