You have mentioned that:
-"I have already sent polite request" hope it by letter under acknowledgment.
-"and they told me it will be given soon", it is believed it is verbally.
You may send a gentle reminder mentioning your representations in chronological order with date time, phone numbers, name of employee contacted by you, and letters supplied by you to good office of your appointing authority, MD, Head-HR, Company secretary, and request to supply by registered post with AD. If it does not fetch you desired results you can approach the labor inspector.
For your next venture your F&F, salary slip of last drawn salary showing DOJ, copy of resignation with acknowledgment, acceptance letter should suffice. You shall have to convince your next employer.