If you have been ill for 7 days or less you don’t need to see a Doctor. You can complete a Self-Certification form yourself. 

Many employers have their own self-certification forms. If your employer doesn’t have its own form you can download the Self Certification Form. Please print it, fill it in and hand it in to your employer. You do not need to see a Doctor.

If you're sick and off work for more than seven days, your employer will probably ask for proof of your illness. Most employers ask for a fit note from your GP.

However, this will also depend on your employer's company policy on sick leave (or sickness absence). This policy should tell you how many days you can be off sick before you need to provide proof of illness or a fit note.

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.