FRANKFURT, Germany Answering critics who said they were running out of ways to promote growth and lending, the European Central Bank and the Bank of England did something Thursday that neither had done before, committing themselves to keeping interest rates low indefinitely.
The bid to reassure investors brought the two central banks into closer alignment with the Federal Reserve, which, under Chairman Ben Bernanke, has adopted a policy of becoming more open about its intentions.
At the same time, they appeared eager to signal that they would not follow the Fed in preparing for a gradual withdrawal of economic stimulus.
Mario Draghi, the president of the European Central Bank, said at a news conference that crucial interest rates would "remain at present or lower levels for an extended period of time." Until Thursday, the central bank had steadfastly refused to pin itself down on future policy.
"It's not six months," Draghi said. "It's not 12 months. It's an extended period of time." Draghi also said that the central bank was signaling a "downward bias" in interest rate policy, meaning further cuts were possible or even likely.
Only hours earlier, Mark Carney, who became governor of the Bank of England on Monday, made a similar break with tradition. The British central bank said in a statement that any expectations that interest rates would rise soon from their current record low level were misguided.
With their promises of easy money stretching toward the horizon, the central bankers offered more certainty to investors at a time when tensions in Europe are rising again. So-called forward guidance is considered one of the tools available to central banks, but it was one the European Central Bank and the Bank of England had not used before.
European markets reacted positively to the announcements, with the FTSE 100 in London closing 3.1 percent higher and the Euro Stoxx 50, a benchmark of eurozone blue chips, climbing 3 percent. (Markets in the United States were closed for the Fourth of July holiday.)
The euro fell sharply, a development that was probably not unwelcome at the European Central Bank, since a cheaper euro makes European products less expensive in foreign markets, feeding exports. The British pound also fell.