In his letter to Modi, the lawyer who is appearing for two NGOs -- Centre for Public Interest Litigation (CPIL) and Common Cause -- in 2G matter and coalgate -- alleged that Sinha was "acting in concert" with the accused and trying to influence or derail the investigation/prosecution of those cases.
He said instances of Sinha's alleged misconduct are given in detail in the latest affidavit filed by him before the Supreme Court.
In the letter, Bhushan has referred to the issues raked up by him in the apex court relating to the "entry register" at Sinha's residence, which he claimed, showed that top executives of companies accused in 2G and coalgate met him frequently.
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Bhushan mentioned names of some high-profile persons associated with one or the other cases who met with Sinha.
In the letter, he said, "When these facts came to light, Sinha got the CBI to issue a statement that these revelations are a lie and no such entry register exists.
"Later, he issued a statement that this is tantamount to an invasion of his privacy and he would sue the NGO, which has raised this issue before the Supreme Court in a PIL, for perjury.
"It is important to take these actions urgently so that the dignity of the post which he is occupying could be restored and also the important corruption cases, which are being handled by the CBI under his supervision, could be saved from getting derailed," the letter said.