New Delhi, Sept. 17 -- The Reserve Bank of India's refusal to relax listing rules for Tata Sons Pvt. Ltd. delivered an outcome long sought by the conglomerate's biggest minority shareholder, though the road to going public is far from smooth.

The RBI's decision came months after reclusive billionaire Shapoor Mistry, who helms the debt-laden Shapoorji Pallonji Group, published an open letter to the regulator seeking a listing of Tata Sons to help unlock value for the group's 18.4% stake. Valued at about $31 billion by Bloomberg Billionaires Index, nearly three-quarters of Mistry's net worth is coming from the Tata shareholding.

Executives close to SP Group also held meetings with Indian government officials in recent months to press thei...