Advise Regarding Notice Priod Issue

Team,
I'm have been working with one the Consultancy firm since Feb, 2014 at their client side. As per offer letter given by employer at time of joining clearly mentions that I have to give 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 manger (from client side) and HR representative on 10th Dec, 2014.
In return of resignation acceptance, both the parties (client and my employer) asking me to serve 45 days’ notice period, saying that they given the commitment to their vendor.
However I have not been provided any prior notification or Addendum letter for policy change before I submitted my resignation letter. They also threatening me that they will not provide me relieving letter and full n final settlement I did not serve complete 45 days’ notice period.
Can you please advise what kind of action I can take against this approach?
Thanks in advance.

Prashant - Either you take up the new assignment without a relieving letter or serve a 45 day notice period, if your new employer is willing to stretch that long.
It solely depends on you.

So basically there is not law where employee can legal action against the unethical employer in India?

So Basically there is no legal action employee can take against the unethical employers in India?
I'm aware that this is basically HR centric forum, but it would be great help if you can provide any government policies from the employee perspective.

Dear All, Wish all for Prosperous New Year. I am urgently recruiting a Renewable energy company in Chennai. I need peoples from DEEE, BE, B.Tech, for project Engineers 2-4 Years Experienced and CRM, BDM in 5-8 Years Experienceand sales Executives 2-4 Years Relevant Experience. Can anyone help to me.

There are certain provisions under shops and establishment Act of certain States providing for giving experience letter to confirmed employees on their leaving the job. In the present vcase, the employer could not have altered the relieving period unilaterally. He can take recourse under the above Act.

Dear Prashant. Remember it is mutual. Whatever the law, try to get 45 days joining period from your new employer.