The company was cut to BB+, one level below investment grade, from BBB-, S&P said in a statement. After the downgrade, S&P withdrew all ratings at the company's request, it said.
Charoen bought F&N, whose businesses range from milk to shopping malls, through his companies, including Thai Beverage and TCC Assets, earlier this year for S$13.8 billion (US$11 billion).
"We view the group as highly leveraged," S&P said in the statement, referring to Thai Beverage, TCC Assets and F&N.
"We evaluate the group's consolidated cash flow adequacy as substantially weaker and its consolidated debt as considerably higher than that of either Thai Bev or F&N on a stand-alone basis."
Thai Beverage dropped 3.1% to 47.5 Singapore cents at the close of trade, the lowest since Sept 4, before the S&P announcement.