A Current Visa in an Expired Passport
If your current visa is in an expired passport and you have subsequently renewed your passport, although you are entitled to remain in the UK legally under the conditions attached to your visa, you may face difficulty in obtaining employment or continuing to work if you already have employment. This is because your employer is under a duty to the Home Office to check that you have the right to work. They are bound by the Employers' Guide to Right To Work Checks which states that you must be able to present "A current passport endorsed to show that the holder is allowed to stay in the UK and is currently allowed to do the type of work in question". If you cannot show this because your visa is in an expired passport then your employer will refuse to employ you and you should not work. If you are already working when you obtain your new passport your employer should take a copy of the new passport and stop you working until you have made an application (see below).
You can make an application to the Home Office to have your visa issued as a separate biometric residence permit (or BRP) so that you meet the requirement in the employers' guidance. The application you need to make to transfer your old visa into your new visa depends on the type of visa you currently have:
Limited Leave to Remain
If you have a temporary visa (limited leave to remain), such as a Tier 4 visa, and the visa vignette (sticker) is in your expired passport, you must make an application for a “Transfer of Conditions,” using Form TOC. The Home Office will effectively issue you the same visa but on a Biometric Residence Permit (BRP or visa card), which will then ensure that your current visa is endorsed on a current document. The International Student Advisers in the Hub can help you with this application and can check and send it for you.
Indefinite Leave to Remain
If you have indefinite leave to remain, and the visa vignette (sticker) is in your expired passport, you must make an application for a “No Time Limit,” using Form NTL. The Home Office will effectively issue you the same visa but on a Biometrics Residence Permit (BRP or, which will then ensure that your current visa is endorsed on a current document.
Your employer will not be able to hire you until you receive your new Biometrics Residence Permit Card.
Can I work once I have made an application?
Once you have submitted your TOC or NTL application your employer can contact the Home Office to verify that the application is in progress. This then covers the employer and allows them to employ you or let you continue work if you were already working for them at the point that you obtained your new passport.
Further details can be found on the Home Office website.