>> Change in notice period amounts to change in service conditions.
If standing orders were applicable and were certified and notice period stated in CSO is desired to be changed, 21 days' notice as provided under section 9-A of Industrial Disputes Act will be necessary by the employer.
So notice of change should have been supplied.
>> The companies are known to incorporate in appointment letter that interpretation of the company shall be final.
Did you tender notice of resignation and if yes what is the notice period stated by you?
From your post it is noted that company has maintained a studied silence and has not asked to serve the notice period of 3 months in writing?
If you had sent the formal communication to some line manager/HR person escalate to good offices of appointing authority, MD, Company Secretary………….in writing under acknowledgement, and seek reply say within next 7 days by redg. post.
The language stated in appointment letter can be the deciding factor…………….
The probation period stated in appointment letter as posted by you is 3 months and this is without any explicit reference to probation period.
Is it stated that service shall be confirmed in writing?
The completion of the probationary period does not automatically amount to confirming the service of the probationer…………….
The function of confirmation implies exercise of judgment by the confirming authority on overall performance and suitability of the probationer which is to be taken into consideration to reach at a definite conclusion as to whether the services of a probationer are confirmed or terminated.
What is the max. period for which probation period can be extended?
After expiry of such period employee (that wants to) may/can claim deemed confirmation.
During probation period employee can not claim any lien on employment thus notice period during probation period should be NIL.
This is also affirmed in Model Standing Orders. (13: Termination of employment.)
Your establishment is manufacturing so the standing orders may apply to it.
You may check if company has its certified standing orders (CSO).
Draft standing Orders shall get certified if these are certifiable.
You may also check if CSO has been extended to your designation? Notice period is part of service conditions and service conditions stated in CSO can not be negated in appointment letter thus if notice period in CSO is NIL it can’t be 1M or 3 M in appointment letter.
Standing orders applicable to the establishment should be displayed at a conspicuous place/notice board and a certified copy should be supplied to employee against nominal charge say Rs.10/-.You may write to concerned personnel in HR or good offices to supply the same and to inform you if any payment is to be made by you.
Employee or any one can approach CO (Certifying Officer- DLC in o/o Labor Commissioner at redg. office location of company) to know applicability of Certified/Model standing orders and can obtain a certified copy of applicable standing orders, against a set fee say Rs.3/page.
If standing orders are applicable but are not certified Model Standing Orders shall apply.
You may consult a competent and experienced labor consultant/service lawyer and you lawyer may ask you a set of structured questions and may opine that you are covered as ‘Workman” as in ID Act or not.
If the good offices also do not respond you have a reason to approach trade unions, Works Committee, Grievance Redressal Cell, Lawyer/law firm, O/o Labor Commissioner, Civil court……………………..
You may also check if the Div/Office where you are located is registered under factory Act or under Shops and Commercial Establishments Act of the State and accordingly you may look into clauses on Notice of Termination by employee in this enactment also and may approach Inspector under this enactment.
Notice period is not more than 1 month in this enactment.
There are threads to suggest that labor commissioner pointed out that notice period of 1M (in case of confirmed employee) as stated in this enactment shall be VALID and 3 months as stated in appointment letter shall be VOID.
https://www.lawyersclubindia.com/forum/Employer-not-relieving-employee-contt-89308.asp#.UkP5C9KAqWM
You may show the job advertisement, job application, interview call letter, selection letter, offer letter, standing orders applicable to the company (certified or model) appointment letter, resignation submitted by you and other formal communications submitted by you, to your lawyer and proceed under expert advise of your lawyer………………………………The lawyer that has seen your entire docs and has analyzed your inputs can advise you the best.