Expressing sadness over RBI Governor Urjit Patel's resignation, former Prime Minister Manmohan Singh on Monday said Patel's sudden decision to demit office is a severe blow to the Indian economy.In a statement, Singh described Patel as an "economist of high repute and also someone who cared deeply about India's financial institutions and economic policy" and wished him success for future endeavours."Dr Patel's sudden resignation, at a time when the Indian economy is faced with many headwinds is very unfortunate and is a severe blow to the nation's economy," the statement quoted him as saying.Earlier in the day, Reserve Bank of India (RBI) Governor Patel announcing his resignation from the apex bank with immediate effect citing personal reasons.Taking a hit at the NDA government, Singh -- a senior Congress leader -- said: "I also sincerely hope that this sudden resignation of the Governor is not a harbinger of the Modi government's attempts to destroy the institutional foundations of India's $3-trillion economy.""There have been apprehensions expressed earlier by the Deputy Governor of the RBI about the government's intent to raid the capital reserves of the RBI for fiscal purposes. I hope the resignation of the Governor is not a sign that this may soon become a reality," he added.Patel's resignation comes in the backdrop of the ongoing tiff between the government and the central bank over the liquidity and credit crunch in the economy that provoked an extraordinary meeting of the RBI board on November 19.