"The RBI has conveyed that it cannot accede to this request as the same is not in conformity with the extant FEMA regulations, and has advised that any such purchase of shares be at current fair value of shares," Tata Sons said in a statement.
Tata Sons had in November last year applied to RBI to purchase DoCoMo's 26.5 per cent stake at Rs 58.045 per share (totalling about Rs 7,250 crore) - half the price which the Japanese firm paid in 2009.
It added: "This issue will now have to be resolved in the arbitration between the parties. Steps towards initiating arbitration have been taken. We will not be able to comment further as the matter is sub-judice."
DoCoMo in July last year announced plans to exit Tata Teleservices, the 7th biggest mobile operator in India.
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Unable to find a buyer, Tata Sons in November applied to RBI to purchase the stake at Rs 58.045 per share - half the price DoCoMo had paid in 2009.
But it could not get the requisite approvals in time and DoCoMo earlier this month dragged Tatas to an international court of arbitration.
The Japanese firm had stated that as per the 2009 agreement that formed Tata Teleservices, DoCoMo can sell the shares if the joint venture fails to meet performance targets in the fiscal year that ended on March 31 last year.