You do not require a doctor's sickness certificate for any illness lasting 7 days or less. 
Your employer may however require you to complete a self-certification form (SC2)
which is available from your employer or on the HMRC website.
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Evidence that you are sick
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If you are sick for more than 7 days, your employer can ask you to give them some form of medical evidence to
support payment of SSP (statutory sick pay).
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Your employer can ask you to confirm that you have been ill. You can do this by filling in a form yourself when you return to work.
This is called self-certification. If you are sick and off work for more than 7 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).
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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, for example 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.
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Statement of Fitness for Work - 'Fit Note'
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The 'fit note' was introduced on 6 April 2010. With your employer's support, the note will help you return to work sooner by providing more information about the effects of your illness or injury.
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For more information see the DirectGov website (where this information was sourced).