The Constitution of India does not mention ranks of ministers like cabinet minister, minister of state or deputy minister. Article 74 mentions the Council of Ministers with the Prime Minister at the head. Third Schedule has forms of oaths or affirmations of office and secrecy for a Minister for the Union. There is no separate oath for Prime Minister. Similarly there are no separate oaths for other ranks of ministers of the Union.
Article 164 mentions the Council of Ministers with the Chief Minister at the head. Third Schedule has forms of oaths or affirmations of office and secrecy for a Minister for a State. There is no separate oath for Chief Minister. Similarly there are no separate oaths for other ranks of ministers of a State.
Sometimes there have been Deputy Prime Ministers and Deputy Chief Ministers. Vallabhbhai Patel, Morarji Desai, Charan Singh, Jagjivan Ram, Yashwantrao Chavan, Devi Lal and Lal Krishna Advani were Deputy Prime Ministers.
How did the ranks of cabinet minister, minister of state with independent charge, minister of state and deputy minister come about? I have no idea. Nowadays according to the best of my knowledge there are no deputy ministers. In the Constitution it is Council of Ministers and not Cabinet.
During Jawaharlal Nehru’s time there were parliamentary secretaries. Rajiv Gandhi revived the practice and for some time Ahmed Patel, Arun Singh and Oscar Fernandes were parliamentary secretaries. In some states parliamentary secretaries are appointed to accommodate MLAs who could not be made ministers.
Did Indians copy from the British? Unlikely. In UK there is Prime Minister, Deputy Prime Minister, and Secretaries for Home, Defence and so on except for Finance for which there is Chancellor of the Exchequer.
Sometimes something unconstitutional is done. If nobody challenges it or the challenge is overruled others do it later on and it becomes a convention. Rajya Sabha and Lok Sabha should have been in session by the end of April 2014 to approve President’s Rule as per Article 352 failing which President’s Rule should have been revoked. Nothing happened. President Pranab Mukherjee did not uphold the constitution. Manmohan Singh and his ministers did not do anything. Nobody challenged the unconstitutional continuance of President’s Rule in Andhra Pradesh.
President and Governors should see that Councils of Ministers function according to Constitution.