The biometric residence permit is proof of the holder’s right to stay, work or study in the UK. It can also be used as a form of identification (for example, if they wish to open a bank account in the UK). The holder is not required to carry their permit at all times, but they must show it at the border, together with their passport, when travelling outside of, and when returning to, the UK.
Employers have a responsibility to check that prospective employees have a right to work in the UK. Foreign nationals issued with a biometric residence permit must show it to employers before they start work. Employers can accept this, alongside other recommended documents, to confirm the identity and right to work of prospective employees.
The UK Borders Agency has published guidance which shows the features of the permits and how to check them. There is also a free online checking service which can be used to check Biometric Residence Permits.