I understand that your company is forcing you to serve full notice period otherwise they will not issue relieving letter. This seems to be unfair and unreasonable on their part. You have every right to resign from your job and join a new company as per your wish and convenience.
There are some legal aspects and options that you should be aware of regarding your situation. Here are some of them:
The company's insistence on you serving the full notice period before releasing the relieving letter might be seen as unjust and unreasonable. Legally, as per the Industrial Disputes Act, 1947, Section 25F, both an employer and employee must provide at least a month's notice, or the equivalent salary, for termination or resignation respectively. This notice period can be altered based on the employment contract or mutual agreement. Any extension of the notice period through a circular without your agreement might not be enforceable. Employers must issue a relieving letter once the notice period or its monetary equivalent is fulfilled. If they don't, they might be legally accountable for breach of contract and harassment. In case of disputes, you have the right to approach legal avenues like sending a legal notice, consulting the labour commissioner, or approaching the labour court. Always consider seeking legal advice from professionals before making a decision.