You will need to deal with this situation very carefully and be certain the employee does not have the right of work before taking further action. You will still need to follow a fair process and ensure that if you are terminating employment that the dismissal falls within the band of reasonable responses and you have a fair reason for dismissal. Going forwards, you should ensure that proper record-keeping and monitoring processes are in place, to identify when immigration permissions will expire, be aware of the circumstances that could lead to an employee losing their right to work in the UK and put procedures in place to prevent illegal working. Please ensure that you contact us for specific legal advice if you find yourself in this situation.