The court had, after a stinging critique of the CBI last week, asked the agency for crucial details of a meeting at Mr Kumar's office a few days before the report was submitted, where the minister allegedly vetted it.
The CBI director will today submit an affidavit, details of which have been accessed by NDTV. The agency is likely to reveal that the Law Minister had at that March meeting disagreed with a line on "rules not being followed in coal allocation" and asked for it to be phrased in the "passive voice to dilute its impact".
He is also said to have argued that this could not be reflected as a finding till the CBI completed a detailed investigation. The Attorney General and the CBI chief were present.
The Law Minister has claimed that the changes he asked for were "suggestions of a minor nature."
Sources say the CBI is also likely to confess that the Attorney General saw the draft amended by the Law Minister and proposed some more changes. Mr Vahanvati has said in the Supreme Court that he did not review the document.
And the CBI is expected to disclose that officials in the Prime Minister's Office too had proposed modifications to the report and that two small changes that they had sought were accepted.
What happens next to the minister and the government's top legal officer is likely to depend on the Supreme Court's reaction to the CBI affidavit.
The government has so far backed Mr Kumar and argued that there was no breach of propriety. "Why can't the Law Minister consult investigative agencies to whom he provides service of law officers? The government may have been under the scanner but we have a right to find out what's happening," said External Affairs Minister Salman Khurshid. (Watch)
But many in the ruling Congress believe the Law Minister's actions make it untenable for him to remain in office.
The scandal around "coal-gate" has proved to be a scalable disaster for the government. The Opposition's demand for the resignation of the Prime Minister and the Law Minister on the grounds that the latter tried to interfere with the CBI's inquiry has paralysed Parliament for several days.