Section 18(1)(a) in The Disputed Elections (Prime Minister And Speaker) Act, 1977
(a)that on the date of his election a returned candidate was not qualified, or was disqualified, to be chosen to fill the seat under the Constitution or the Representation of the People Act, 1951, (43 of 1951) or this Act or the Government of Union Territories Act, 1963; 20 of 1963 or