If unauthorised annual holiday is taken the employer may take disciplinary action.
If an employee is ill during a holiday period, they may be able to regain any days of holidays lost on which they were sick to be taken at a later date. Any such period of sickness will have to be proven and would be subject to the usual rules governing the entitlement to Statutory Sick Pay, i.e. for a seven day period of sickness a fit note would be required.
If it is not possible to take all of the regained holiday during the same holiday year, then an employee should be permitted to carry holidays lost due to sickness absence forward into the next holiday year (see Holiday carry over).