February's downgrade by Moody's, another of the big three rating agencies, was seen as a political embarrassment to the government, which had promised to protect the UK's credit rating when it was elected in 2010.

BBC: S&P reaffirms UK triple-A rating

The government has arranged it so there are just three sources that really matter for bond ratings the big-three credit rating agencies are effectively a government-approved oligopoly.

FORBES: At Odds Over S&P