Your first task is to report the lost card. It is not absolutely necessary to visit a job centre in order to apply for a replacement card. Luckily, you can do this by simply calling the National Insurance Registration Helpline (0845 915 7006). They will then mail a new card to your home address. Keep in mind, however, that you will only be provided with one replacement. This means that if you lose the card again, they will not send you a new one.