Subbarao, who was the Finance Secretary from April 2007 to September 2008, is a key witness in the case in which former Telecom Minister A Raja and others are facing trial.

During the recording of his statement, Subbarao told the court he had written a letter on November 22, 2007 to the then Telecom Secretary D S Mathur in which he had questioned the spectrum fee of around Rs 1,600 crore for pan-India licence.

He said that he had also questioned as to how the spectrum fee of Rs 1600 crore, which was fixed in 2001, could be applied in 2007.

"I also questioned how the rate of Rs 1,600 crore, determined in as far as back in 2001, could be applied for licences given in 2007...," he told Special CBI Judge O P Saini.

CBI prosecutor A K Singh also showed various files and notes of various departments concerned, including that of Department of Telecom and Ministry of Finance, to Subbarao during the recording of his statement which is likely to continue throughout the day.

Till November 11, the court had recorded statements of 77 CBI witnesses in the case in which Raja is the key accused.

Besides Raja, DMK MP Kanimozhi, former Telecom Secretary Siddharth Behura, Raja's erstwhile private secretary R K Chandolia, Swan Telecom promoters Shahid Usman Balwa and Vinod Goenka, Unitech Ltd MD Sanjay Chandra, three top executives of Reliance Anil Dhirubhai Ambani Group (RADAG) -- Gautam Doshi, Surendra Pipara and Hari Nair -- are facing trial in the case.

Directors of Kusegaon Fruits and Vegetables Pvt Ltd Asif Balwa and Rajiv Agarwal, Kalaignar TV Director Sharad Kumar and Bollywood producer Karim Morani are also accused in the case.

Besides these 14 accused persons, three telecom firms – Swan Telecom Pvt Ltd (STPL), Reliance Telecom Ltd and Unitech Wireless (Tamil Nadu) Ltd -- are also facing trial in the case. They have been arrayed as accused in the first two CBI charge sheets filed on April 2 and 25 last year respectively.

The court had on October 22, 2011 framed charges against 17 accused under various provisions of the IPC and the Prevention of Corruption Act dealing with offences of criminal conspiracy, cheating, forgery, faking documents, abusing official position, criminal misconduct by public servant and taking bribe.