Before your employer can decide on your entitlement to Statutory Sick Pay (SSP), you must tell them that you are sick. You will also have to provide your employer with some form of medical evidence.
Your employer may have their own rules about when and how you tell them you are sick and should tell you what these are. Please check with your employer if you do not know. Your employer does not have to pay SSP for any days before you tell them if you do not stick to these rules.
If your employer does not have their own rules, you should tell your employer within seven days of the first day that you are sick. If you delay telling your employer they do not have to pay SSP for any days before you tell them.
However your employer cannot insist that you tell them:
Your employer cannot ask you to provide evidence that your are sick for the first seven days of illness. They may ask you to fill in a self-certificate of their own design or form SC2 which you can get from your GP's surgery, or from the HM Revenue and Customs (HMRC) website.
If you are sick for more than seven days, your employer can ask you to give them some form of medical evidence to support payment of SSP.
It is up to your employer to decide whether you are incapable of work. A medical certificate from your doctor is strong evidence that you are sick and would normally be accepted, unless there is evidence to prove otherwise.
You could also provide evidence from someone who is not a medical practitioner, e.g. a dentist. Your employer will decide whether or not this evidence is acceptable. If your employer has any doubts, they may still ask for a medical certificate from your GP.