I'm have been working with one of the consultancy firms since February 2014 at their client site. As per the offer letter given by my employer at the time of joining, it clearly mentions that I have to give an official notice of 15 days when resigning from the job. Now, I have resigned from my position and sent official mail communication to my reporting manager (from the client side) and HR representative on December 10, 2014. In return for my resignation acceptance, both parties (client and my employer) are asking me to serve a 45-day notice period, saying that they have given a commitment to their vendor. However, I have not been provided any prior notification or addendum letter for a policy change before I submitted my resignation letter. They are also threatening me that they will not provide me with a relieving letter and full and final settlement if I do not serve the complete 45-day notice period. Can you please advise what kind of action I can take against this approach? Thanks in advance.