The principle that the framing of the new Constitution for independent India should be primarily (though not solely) the responsibility of Indians themselves, was for the first time conceded in the

The principle that Indians should primarily frame their own constitution was first conceded by the British government in the 'August Offer' of 1940 [1][2]. Issued by Viceroy Lord Linlithgow, the offer proposed the setting up of a constituent assembly after World War II, where Indians would decide the constitution according to their own social, economic, and political conceptions [4]. While the Indian National Congress had officially demanded such an assembly in 1935, it was only in 1940 that the British finally accepted this demand in principle to secure Indian cooperation during the war [1]. Although the Cripps Proposals of 1942 and the Cabinet Mission of 1946 further elaborated on the mechanics of this process, the initial concession of the 'responsibility of Indians themselves' originated with the August Offer [1][4].

Sources

1. [1] Indian Polity, M. Laxmikanth(7th ed.) > Chapter 2: Making of the Constitution > DEMAND FOR A CONSTITUENT ASSEMBLY > p. 11
2. [2] Laxmikanth, M. Indian Polity. 7th ed., McGraw Hill. > Chapter 2: Making of the Constitution > DEMAND FOR A CONSTITUENT ASSEMBLY > p. 11
3. [4] https://en.wikipedia.org/wiki/August_Offer