Members can not judge as to whether a document is fake or genuine.
If the official that issued relieving order states officially to your employer, that it was not issued by him, it is deemed to be fabricated and forged until some one establishes that it is genuine.
File a Police complaint and also send the copy of such relieving order to TRUTH LABS for confirmation of signature.
Why should past employer say that and what is the personal benefit for him to deny the signature is not known to members. But if the relieving order is really fake, they can even file a complaint against such ex-employee for fabricating and forging such relieving letter making the case more complicated as they are having such fake letter in their possession.
The issue raised by you in your query is as to whether Present employer can ask you to resign within four months and there was no query on your past employment at all.
The pointed guidance is that an employer can even dismiss/terminate the employee if it is known that employment is secured employment on false documents.
Where is the mistake and who is really guilty whether past employer, employee or present employer has to be decided by investigating agencies and not members?
Members can only presume facts basing on one sentence queries and facts are not known to them.