Is it mandatory to give the new company my old relieving letter on the day of joining itself?

I work for Company A in India and am in my notice period. In about 2 weeks from now, I will be joining Company B.

Company A, however, says that they have a policy that they will give a relieving letter along with full and final settlement only 45 days after the last date. They say they will try to expedite, but that is the policy.

Some questions:
1. I have requested them for a relieving letter on the last day itself. Can HR legally deny that? - as long as I didn't commit a crime.
2. Is it mandatory to give the new company my old relieving letter on the day of joining itself? Or can that be deferred by a few days or weeks?

Note: The offer letter/contract I signed does not state anything about these things at all.

There are no universal rules in this regard. Every organization has its own rules regarding this. If they say 45 days is the rule, there is hardly anything you can do about it. Generally, a relieving order is issued at the earliest, and the experience certificate is given later along with the F&F settlement on submission of a No Dues Certificate.

Though a 45-day gap may seem unreasonable and illogical, some delay is inevitable. However, it is not illegal as there is no legal provision for it. This delay may be serving them as part of a retention strategy.

Generally, the offer letter will list out the documents to be submitted along with the joining letter. If they are unable to provide you with the relieving order or experience certificate, request them to give you a letter indicating the acceptance of resignation and the date of being relieved from service.

So far as a relieving letter is concerned, I have a different perception. When an employee is relieved from his job in an organization for whatever reason, the relieving letter or order indicates his formal separation from the organization. When such formal separation is a consequence of no pending liability on the part of the employee after a thorough verification process, how can the employer withhold it? When the employee is physically permitted to walk out, it is incumbent upon the employer to place it on record with the issuance of a relieving order forthwith.