In the letter to the government, CBI chief Alok Verma had called Rakesh Asthana "a source of demoralisation, subject of probes," sources said. The CBI has refused to comment on the matter.

Mr Asthana, a Gujarat officer dubbed "the PM's blue-eyed boy" by Congress president Rahul Gandhi in a tweet on Monday, was elevated as one of the country's top investigators in controversial circumstances last year. Sources in the PM's office say the CBI has not sought permission to act against him, as is required in such cases.

On Monday, the CBI arrested its officer Devender Kumar, who worked with Rakesh Asthana, and accused him of making false statements against its chief. In an unprecedented fallout of the fight, the CBI searched its own offices - specifically the rooms of senior officers who have worked with Rakesh Asthana.

The CBI's First Information Report (FIR) against Rakesh Asthana is based on the claims of Sathish Sana, a Hyderabad-based businessman investigated in a money-laundering case against meat exporter Moin Qureshi.

Sathish Sana has alleged that he was asked to pay a bribe of Rs. 5 crore to Mr Asthana to be spared. The CBI says Sathish Sana told a magistrate that he had paid Rs. 2 crore as bribe to Mr Asthana over a 10 month period from December last year.

Manoj Prasad, a Dubai-based investment banker, was allegedly the middleman who organized the bribe. He was arrested on October 16 when he was allegedly about to collect an installment. The CBI claims nine phone calls made by Manoj Prasad's brother Somesh immediately after the arrest included conversations with Mr Asthana.

Somesh Prasad has also been charged by the CBI. Manoj and Somesh are the sons of a former officer of the Research and Analysis Wing (R&AW). The CBI says the role of R&AW officials is also being investigated.

In a complaint to the government in August, Rakesh Asthana had alleged it was the CBI chief who took that bribe but had framed him, because he prevented Sathish Sana from skipping the country. Earlier this month, Mr Asthana also accused the CBI chief of trying to frame him in the case of Gujarat-based company Sterling Biotech, which is being investigated for huge loan default.

Sathish Sana had, during his interrogation, "admitted" to bribing the CBI director, Mr Asthana said in his letter to the Cabinet Secretariat. The letter also listed 10 instances of corruption, criminal misconduct, interference in the investigation of the sensitive cases. The claim has been termed fraudulent by the CBI and for this it has arrested Devender Kumar.