You do not require a medical certificate, now called a 'Statement of Fitness for Work’ (known as a 'fit note') for any illness causing absence from work for a period lasting seven days or less.
Evidence that you are sick 
If you are sick for seven days or less your employer can ask you to confirm that you've been ill. You can do this by filling in a form yourself when you return to work. This is called self-certification. Your employer may have their own version of the self certificate form for you to complete. Otherwise, form (SC2) is available from your employer or on the HMRC website.
If you are sick and off work for more than seven days, your employer can ask you to give them some form of medical evidence to support payment of SSP (statutory sick pay). They 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.
Should your employer insist that you are issued with a 'fit note' for a period of sick leave lasting seven days or less this is likely to attract a fee from your GP.
Statement of Fitness for Work - ’Fit Note'
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.
The dates on the 'fit note' are inclusive (for example: a 'fit note' dated from 2 April to 10 April will no longer apply from 11 April).
The 'fit note' belongs to you, the patient, and you should retain it. Your employer(s) will probably take a copy for their own records.
Your doctor cannot forward date a 'fit note (ie cannot date it to start in the future) but may be able to back date it (ie cover days already past).
Going back to work
You should go back to work as soon as you feel able to and with your employer’s agreement - this may be before your 'fit note' runs out. You do not need to go back to see your doctor before going back to work. This applies even if they have asked to assess you again but please do attend if you feel it is necessary. Your doctor cannot give you a 'fit note' stating that you are ‘fit for work’.
For more information see A guide for patients and employees This is part of the DirectGov website (where this information was sourced)