Moody's said the downgrade reflects "heightened concerns regarding NTL's liquidity position and longer-term ability to adequately service its debt". It added: "A restructuring of the company's balance sheet is a greater possibility."
The agency retained a negative outlook on the stock.
Earlier this month, NTL reported widened third-quarter losses to £704m as it wrote off the value of assets and interest payments on its debt mountain rose.
The company has put a freeze on acquisitions, cut staffing levels and has put its TV transmission business up for sale in a bid to reassure investor concerns which surged because of concerns of a funding gap before NTL breaks even.
NTL described the downgrade as a "disappointment".
"The company and its management have for some time been engaged in making adjustments to its business plan to reflect the current and anticipated business environment," NTL said in a statement.
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