What can i do if builder is not ready to give possession letter

My flat was registered under Maharashtra Ownership Flats Act 1963 in Sept 2014 with a clause for formation of co-op society.Builder issued me a letter in Dec 2016 enclosing therewith building completion certificate dated Apr 2016 issued by the Municipal authorities. In the said letter, I was asked to pay balance sum as per terms of agreement and I had paid the same in Dec 2016. I had also paid one year maintenance as per agreement in Feb 2017. Builder has provided the keys of flat. As builder have not issued POSSESSION LETTER, I have neither shifted into the flat nor rented it out. It is kept vacant. Despite repeated requests, builder has been dilly-dallying the issue of POSSESSION LETTER for last 15 months. In Jul 2017, Builder arbitrarily registered DEED OF DECLARATION under Maharashtra Apartment Ownership Act 1970 & has been insisting that I should submit DEED OF APARTMENT. What remedy is available for me to force builder to issue possession letter ?