Chandrachur Ghose who runs the website www.Subhaschandrabose.Org had appealed under the RTI Act to provide access to letters written by Netaji's widow and daughter to the government.
"The three files in question are classified and disclosure of the documents contained in them would prejudicially affect relations with foreign countries," the PMO said in its reply.
Signed by Rajeev Topno, director (PMO), the response said these files were therefore exempt from disclosure under Section 8(1)(a) read with section 8(2) of the Right to Information Act, 2005.
Ghose, who is writing a biography on the leader, said the life and times of Netaji, who had disappeared in 1945, is a mystery since many files relating to it are held by various government departments.
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In an earlier disclosure under the RTI appeal made by 'Mission Netaji', a Delhi-based research trust, the PMO had admitted that it was holding 33 secret files on Netaji.
"The only reason why the Government is able to keep files about Netaji secret in this age and time is that there is no one to apply pressure on it. No political party of consequence or a state government stands up for Netaji today, unlike other national icons such as Bhagat Singh, Sardar Patel and Babasaheb Ambedkar," said Anuj Dhar, author of two books on Netaji.
The Mukherjee Commission had rejected the opinion that he died in a plane crash in Taiwan on August 18, 1945.