I am an employee at company A for more than a year now. Company A has a majorly profitable client (Company B) and I have been sent to handle the projects at B. The relation between A and B is contract-bind which is renewed after every 3 months. I had to sign the contract saying that I am the employee who has been dispatched for the client in that period of time, each time it gets renewed.
Company A pays me salary and all my terms are according to the contract which I signed at the time of joining. The contract with A states that my notice period is of 1 month.
Recently, I have been offered the a better position. Therefore, I slid the notice at A that I would be leaving in a month on 11th of Oct. Yesterday, I got a call stating that it would not be possible for me to leave as the contract with B ends in December. (I did not sign on the contract between A-B as a medium for Oct-Dec)
How to enforce my statement that I cannot work till December? Is it legal/ethical of what A is doing here? Any help is appreciated. Thanks in advance.
UPDATE: After the discussion with company A, they agreed with the 1 month notice period and are replacing my position with another employee for the contract duration as @Smock mentioned in the comments.