Greece's credit ratings were cut to "Selective Default" by Standard & Poor's after it negotiated the biggest sovereign debt restructuring in history.

S&P dropped Greece's rating from CC, two levels above default, after the government added clauses to its debt designed to mop up investors unwilling to take part in the exchange, the New York-based company said in a statement today.

The downgrade follows a reduction last week by Fitch Ratings to C, while Moody's Investors Service has said it will cut the nation to its lowest rating. Greece published the formal offer document last week for its agreement to exchange bonds for new securities, with investors taking a haircut of 53.5 percent. The restructuring uses so-called collective action clauses to discourage holdouts, the use of which would trigger credit-default swap insurance contracts on the nation's debt, according to the rules of the International Swaps & Derivatives Association.