If you have settled status in the UK, such as indefinite leave to enter or remain, then you will not require a certificate of approval to give notice to marry to a Registrar.
If you do not have settled status in the UK, you will need to apply for a certificate of approval from the Home Office.
To qualify for a certificate of approval, you must have been granted leave to enter or leave to remain for more than six months on your current visa, and three months of that leave must still remain. If you do not qualify, you will need to return to your country of origin, or the country in which you are normally resident, and apply for a visa there.
The certificate of approval application form is now available on the Immigration & Nationality Directorate (IND) website: www.ind.homeoffice.gov.uk. You can ask for an application form to be sent to you by telephoning 0870 241 0645.
All applications have to be made by post, as you will not be able to apply in person at IND's Public Enquiry Offices for this service. The application form has details of the documents you need to send with the form, and where you should send it to. A fee of £135 is payable for this service. Details of the ways you can pay are also included in the application form. We recommend that applications are sent by Royal Mail Recorded Delivery. You should allow between 3 and 13 weeks for your application to be processed.
The certificate is valid for three months from the date it is issued, or the date when you leave to enter/remain expires, if this is less.
If both people getting married are subject to immigration control, they both need to make separate applications and each pay a fee for their certificates of approval.
The Home Office will not re-issue a replacement certificate if you lose the original. You will have to make a new application.
Now read - Giving notice to marry at a Register Office.
Previous: I want to get married in the UK, and I am overseas | Next: Giving notice to marry at a Register Office