Continue reading the main story
The Federal Reserve has said it does not now expect to raise interest rates in the US until late 2014.
The surprise move sent the dollar sharply lower in markets, and caused US government borrowing costs to fall.
In its regular policy statement, the central bank said it saw "significant downside risks" to the economy, and said inflation had fallen back to a level in line with its mandate.
Rates remain in a target range of zero to 0.25%.
The Fed did not announce any new quantitative easing measures - pumping newly created money into the financial system by buying up US government debt and other assets.
However, it repeated a concern expressed in previous statements that the US economy faced "significant downside risks" and that it "expects to maintain a highly accommodative stance for monetary policy".
Unlike in previous statements, the Fed also no longer said it was monitoring inflation closely, reinforcing the impression that prices rises - which had been some way above the Fed's informal 2% target last year - were no longer a major concern.
Dollar
The interest rate announcement caught markets by surprise. They had been expecting rates to start rising from mid-2014, instead of the late 2014 forecast by the Fed.
The dollar fell, as the lower expected interest rates made the currency a slightly less attractive place to keep cash.
The euro rose one-and a-quarter cents to $1.31 following the release of the statement, which came at the end of a regular two-day meeting of the Fed's policy-setting committee. The pound rose just under a cent to $1.564.
Meanwhile, the US government's implied cost of borrowing in markets for 10 years fell from 2.06% to 1.94%, as traders priced in the lower medium-term interest rate expectations.
The Fed first began stating its expectations about how long it expected interest rates to remain at their historic low back in August last year, as a way of lowering longer-term interest rates and thereby stimulating the economy.
Until now, it had been promising to hold rates down until mid-2013.
In another novel move, the Fed also plans to reveal the interest rate forecasts of individual committee members later on Wednesday.