The agency said lowering of oil price projections have also hurt the Anil Agarwal-led firm's cash flows.
"We downgraded Vedanta Resources because we expect the company's financial performance to remain weak for the next 12-18 months," S&P's Credit Analyst Mehul Sukkawala said in a statement.
Low commodity prices, including the agency's recent lowering of oil price forecast, have hurt the London Stock Exchange-listed company's cash flows, which were already stretched by the high debt levels, he added.
"The outlook is negative. At the same time, we lowered our long-term issue ratings on the notes and loans that the company issued or guarantees to 'B+' from 'BB-'. We removed all the ratings from CreditWatch, where they were placed with negative implications on September 9, 2015," it said.
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On negative outlook, Sukkawala said it reflects the risk that Vedanta Resources' financing for its upcoming maturities could be delayed.
"It also reflects the risk of commodity prices remaining weak for a prolonged period, resulting in the company's financial ratios not recovering in line with our expectations of FFO cash interest coverage rising above 1.75x in fiscal 2017," he added.
Vedanta Resources is dependent on banks to roll over its short-term debt and to help refinance the USD 2 billion of maturities in June/July 2016, S&P said.
"We believe Vedanta Resources has decent banking relationships and Vedanta Ltd has good credit standing in India, which will help Vedanta Resources roll over its short term debt and meet its near-term refinancing requirements," it added.
"We also anticipate that Vedanta Resources will extend a USD 1.25 billion loan from Cairn India due in mid-2016," S&P said.