Pakistan and India came into being on 15 August 1947.

The Indian Independence Act 1947 was an Act of the Parliament of the United Kingdom that partitioned British India into the two new independent dominions of India and Pakistan.

The Prime Minister of the United Kingdom, Clement Attlee, announced on 20 February 1947 that:

1. British Government would grant full self-government to British India by June 1948 at the latest,

2. The future of Princely States would be decided after the date of final transfer is decided.

The Act received the royal assent on 18 July 1947, Pakistan and India came into being on August 15, as two new countries.

The legislation was formulated by the government of Prime Minister Clement Attlee, after representatives of the Indian National Congress, the Muslim League, and the Sikh community came to an agreement with the Viceroy of India, Lord Mountbatten of Burma, on what has come to be known as the 3 June Plan or Mountbatten Plan.

3 June Plan was also known as the Mountbatten Plan. The British government proposed a plan announced on 3 June 1947 that included these principles:

1. Principle of Partition of India was accepted by the British Government

2. Successor governments would be given dominion status

3. Implicit right to secede from the British Commonwealth

The Indian Independence Act 1947 was the implementation of June 3 Plan.

The Act's most important provisions were:

• The cement of the office of Governor-General in each of the two new countries, as representative of the Crown;

• The conferral of complete legislative authority upon the respective Constituent Assemblies of the two new countries;

• The termination of British suzerainty over the princely states, with effect from 15 August 1947, and recognized the right of states to accede to either dominion

British India had 17 provinces and 562 princely states. Upon the Partition of British India into the Dominion of India and Dominion of Pakistan according to the Indian Independence Act 1947, 11 provinces; 1. Ajmer-Merwara-Kekri 2. Andaman and Nicobar Islands 3. Bihar 4. Bombay 5. Central Provinces and Berar 6. Coorg 7. Delhi 8. Madras 9. Panth-Piploda 10. Orissa 11. United Provinces were included in India, 3 provinces; 1. Baluchistan 2. North-West Frontier 3.  Sindh was included in Pakistan, and 3 provinces; 1. Punjab 2. Bengal 3. Assam was partitioned between India and Pakistan.

On 4 June 1947, Viceroy of British India, Lord Mountbatten held a press conference in which he addressed the question of the princely states, of which there was then a total of 562. The treaty relations between Britain and the Indian States would come to an end, and on 15 August 1947, the suzerainty of the British Crown was to lapse. Consequently, the princely states would assume independent status. They would be free to choose to accede to one or the other of the new dominions. 

Out of 562 princely states of British India, 547 joined India and 11 states; Amb, Chitral, Dir, Phulra, Swat, Bahawalpur, Khairpur, Kalat, Kharan, Lasbela, Makran, signed an instrument of accession to join Pakistan. 

The states of Junagadh, Manavadar, and Hyderabad, with majority Hindu populations but with Muslim rulers, were annexed to India after military actions by Indian Army soon after Lord Mountbatten left India in 1948. 

The state of Jammu and Kashmir, which was expected to accede to Pakistan on account of its 77% Muslim majority and its cultural and commercial links to West Punjab (Pakistan), but whose Hindu ruler chose to accede to India, became a disputed territory.