The senior Congress leader has alleged that a meeting of a committee that chooses the CBI chief - made up of the Prime Minister, the Chief Justice of India and the leader of the largest opposition party in the Lok Sabha, in this case Mr Kharge - was deliberately not called to "facilitate giving the charge to a junior officer."

Rakesh Asthana, reportedly the government's top choice for the post, was, as a 1984 batch Indian Police Service or IPS officer, too junior to even make it to a panel of possible candidates for CBI chief to replace Anil Sinha who retired last week.

Critics allege that by appointing Mr Asthana as interim chief, the government not only circumvented the need to call a meeting of the selection committee, but also built an opportunity to make the officer CBI chief next year, when he will be senior enough to be part of the panel of candidates.

Reacting to Mr Kharge's letter to the PM, Union Minister Kiren Rijiju said, "Because he was from Gujarat cadre his credentials were not looked into? How can we give a colour of an officer by clubbing him with some state, cadre or something like that?"

In his letter to the Prime Minister, Mr Kharge has also referred to the sudden transfer of RK Dutta, the most senior officer in line to be the next CBI chief, only three days before Mr Sinha retired.

Mr Dutta, who was transferred to another department in the home ministry with a special post created for him, was in charge of two high-profile corruption cases - the coal allocation case and the 2G case.

Bureaucrats had expressed surprise at Mr Dutta's transfer as the Supreme Court had ordered that no officers connected with the two cases should be removed.

But government sources have argued that Mr Dutta was not an investigating officer in the cases and had a supervisory role and so his being moved out would not impact the cases.

The controversy has reached the Supreme Court via a petition challenging Mr Asthana's appointment filed by lawyer and activist Prashant Bhushan, which the court has agreed to hear. The petition too raises questions on a meeting of the selection committee not being called to decide on the next CBI chief and Mr Dutta's transfer days before the post fell vacant.