If you are off sick, your employer is entitled to demand a doctor's certificate, regardless of the duration of absence. Apart from anything else, they would be failing in a duty of care if they allowed you back to work if you were still ill. If the employer has reason to believe that the absence is not down to sickness - possibly because of the time of year, alignment with majot sporting occasions etc, they may decide that self certification is not appropriate and ask for formal medical evidence of any claimed illness. Sadly, in a number of areas of industry, the habit of 'pulling a sickie' when individuals are not enamoured with rostered duties and possibly have no annual leave remaining, has been common place. Some companies are better than others at managing this behaviour. It is worth remembering that such behaviour is actually misconduct and can lead to the employee being sacked.