You do not require a doctors sickness certificate for any illness lasting seven 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
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.
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.
For more information see the DirectGov website (where this information was sourced)