It is not the whole Act which would be held invalid by being inconsistent with Part III of the Constitution but only such provisions of it which are violative of the fundamental rights, provided that the part which violates the fundamental rights is separable from that which does not isolate them. But if the valid portion is so closely mixed up with invalid portion that it cannot be separated without leaving an incomplete or more or less mingled remainder the court will declare the entire Act void. This process is known as doctrine of severability or reparability.

The Supreme Court considered this doctrine in A.K. Gopalan v. State of Madras, A.I.R. 1950 S.c. 27 and held that the preventive detention minus section 14 was valid as the omission of the Section 14 from the Act will not change the nature and object of the Act and therefore the rest of the Act will remain valid and effective. The doctrine was applied in D.S. Nakara v. Union of India, AIR 1983 S.C. 130 where the Act remained valid while the invalid portion of it was declared invalid because it was severable from the rest of the Act. In State of Bombay v. F.N. Balsara,A.I.R.l.951 S.C. 318 it was held that the provisions of the Bombay Prohibition Act, 1949 which were declared as void did not effect the validity of the entire Act and therefore there was no necessity for declaring the entire statute as invalid.

The doctrine of severability has been elaborately considered by the Supreme Court in R.M.D.C. v.Union of India, AIR 1957 S.c. 628, and the following rules regarding the question of severability has been laid down :
(1) The intention of the legislature is the determining factor in determining whether the valid paIt of a statute are severable from the invalid parts.
(2) If the valid and invalid provisions are so inextricably mixed up that they cannot be separated from the another, then the invalidity ofa portion must result in the invalidity of the Act in its entirety. On the other hand, if they are so distinct and separate that after striking out what is invalid what remains is itself a complete code independent of the rest, then it will be upheld notwithstanding that the rest had become unenforceable.
(3) Even when the provisions which are valid, are distinct and separate from those which are invalid if they form part of a single scheme which is intended to be operative as a whole, then also the invalidity ofa part will result in the failure of the whole.
(4) Likewise when the valid and invalid parts of a Statute are independent and do not form part of a Scheme but what is left after omitting the invalid portion is so thin and truncated as to be in substance different from what it was when it emerged out of legislature, then also it will be rejected in its entirety.
(5) The severability of the valid and invalid provisions of a Statute does not depend on whether provisions are enacted in same section or different section, it is not the form but the substance of the matter that is material and that has to be ascertained on an examination of the Act as a whole and of the setting of the relevant provisions therein.
(6) If after the inval id portion is expunged from the Statute what remains cannot be enforced without making alterations and modifications therein, then the whole of it must be struck down as void as otherwise it will amount to judicial legislation.
(7) In determining the legislative intent on the question of severability, it will be legitimate to take into account the history of legislation, its object, the title and preamble of it

End-Notes
Sources:
http://www.legalserviceindia.com/constitution/const_home.htm