Now, regulators are stressing that the benchmark could be gone by 2021.

What will replace it? Nobody knows for sure.

As traders speculate about what will happen to financial markets when Libor disappears, regulators appear to be worried that banks are not taking the coming change seriously enough. To move away from Libor requires vast amounts of work, and given how tightly woven it is into the financial fabric, it isn’t the kind of thing anyone wants to see rushed.

“I hope it is already clear that the discontinuation of Libor should not be considered a remote probability,” Andrew Bailey, chief executive of Britain’s Financial Conduct Authority, said in a speech last week.

Warning against “misplaced confidence in Libor’s survival,” Mr. Bailey said that the number of financial contracts with interest rates derived from the benchmark continued to grow. Regulators in the United States, including the chairman of the Commodity Futures Trading Commission, have raised similar warnings.

On Thursday, a meeting that is scheduled at the Federal Reserve Bank of New York will focus on preparing for a world without Libor: A panel is expected to address crucial details, including the best ideas for language that should be used to replace Libor in contracts for financial products like business loans, derivatives and floating-rate notes.