You do not require a doctor's 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.
Sick notes can only be issued when they are due. Although they can be back-dated, they cannot be future-dated.
Evidence that you are sick
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 (statutory sick pay).
It is up to your employer to decide whether you are incapable of work. A medical certificate, now called a 'Statement of Fitness for Work’ (see below) from your doctor is strong evidence that you are sick and would normally be accepted, unless there is evidence to prove otherwise.
Hospital Discharges
If you have been discharged from the hospital, your hospital team will issue any fit notes for the entire period of your recovery. Please contact the ward staff (even after discharge) if this has not been done.

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Statement of Fitness for Work - 'Fit Note'
Sick notes should only be issued for people that are incapable of all work. If you feel that you are capable of some work but not your usual duties, you could consider this with your employer and they may be able to accommodate changes to your work or role to keep you working. When you consult us about a medical problem that affects your fitness to work we may consider this option with you. We can request your employer make some adjustments in a ‘May be fit for work’ note. This may allow you to get back to work sooner and support your recovery. If your employer cannot support the changes we request to keep you at work, they can just treat the note as a ‘sick note’ to allow you to be off work and support the payment of statutory sick pay.
For more information see the DirectGov website (where this information was sourced).
Extending sickness certificates
If you have a sickness certificate which is running out but you feel you are still not fit to return to work, you will need to contact our admin team within 5 working days of your current note running out. This will allow your GP to arrange a review with you if needed.
Renewal requests for sickness certificates will be processed by the doctor within 5 working days.