Is NOC required from existing employer for going for interview

Sir, I am a permanent central government employee as my probation period has already completed. I have applied for a recruitment for assistant registrar post in IIT but I forgot to intimate to my parent government organization as at the time of applying, I couldnt process my application through proper channel however I submitted my advance copy to IIT. Sir, now I have been shortlisted for the same and called for interview. and accordingly I intimated my parent govt office and requested for providing NOC so that I can attend the interview. But my parent govt office is denying the same by stating the reason that I didnt take prior permission before applying the above post so the question of issuing NOC does not arise. sir please help me in this regard, is there any rule or judgement in this regard so that My govt office HOD can issue the NOC. please help me as I am in urgent need.