Wall Street markets are expected to tumble at the opening bell later after rating agency Fitch's move to downgrade the US government's credit rating. Fitch downgraded the United States to AA+ from AAA, pointing to fiscal deterioration over the next three years as well as a growing general government debt burden, making it the second major rating agency after Standard & Poor's move in 2011 to strip the country of its triple-A rating. The yield on US 10-year Treasury notes slipped to 4.02pc while safe havens like gold and the Japanese yen rose. Megacap stocks including Tesla, Nvidia, Meta and Microsoft fell between 0.8pc and 2.5pc in premarket trading. Futures on the Dow Jones Industrial Average were were down 0.3pc, while the S&P 500 had fallen 0.5pc. The Nasdaq 100 is on track to open 0.7pc lower.