"The pressure on the US rating is increasing, " said Fitch's David Riley, adding that if there was no deal, there was a "material risk" of a downgrade.
Fitchsaid after Mr Osborne's announcement that the government's policy framework had been a key factor helping the UK to hold on to to its top AAA credit rating, and the loss of the debt rule had weakened both the coalition's credibility and the rating.
Late Wednesday afternoon in Europe, Fitch Ratings said it had cut Iceland's long-term sovereign currency issuer default ratings to "BBB-, " from "A, " putting them firmly in "junk" territory.