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The only circumstance under which your employer is legally obliged to allow you paid time off work to attend interviews is if you are facing redundancy and you have worked for your employer for at least two consecutive years. In this case, you are entitled to a reasonable amount of time off work (during the period leading up to your redundancy) to look for alternative employment.
If you work flexible hours you may be able to work this to your advantage and arrange your interviews to fit in with your working hours.
In the event that you are applying for an internal promotion, or an alternative position within the same company, your employer may, at their discretion, allow you to attend the interview during work time.
If you do not work flexible hours, are not on notice of being made redundant and are not applying for an internal position, it may be necessary for you to take annual leave in order to attend interviews. This way you should not have to give your employer an explanation as to why you require time off work (you may not wish to make your employer aware that you are seeking alternative employment) and you will not lose out financially by taking annual leave to attend interviews. If you do not have enough annual leave then you may have to take unpaid time off work to attend interviews and you may have to explain to your employer why you require this time off.
Many prospective employers understand that you are not in a position where you wish to inform your current employer that you are seeking alternative employment and they are usually, therefore, flexible as to when you may be able to attend an interview (for example, they may agree to you attending an interview after outside of normal working hours if this is mutually convenient).