The fate of Libor, the rate that underpins $350 trillion worth of financial contracts, has been hanging in the wind for years. Now its future is clear: The U.K.'s Financial Conduct Authority (FCA) says Libor will cease to exist by the end of 2021. And that means a lot of work for corporate treasurers, who are responsible for all those loan agreements, derivatives contracts, and other documents that refer to Libor.

The reputation of Libor, the rate at which major banks lend to one another, was tarnished by a 2012 scandal that revealed banks had manipulated the rate. But regulators' move to pull the plug has more to do with the fact that in the wake of the financial crisis, interbank lending tailed off, making the bank quotes on which Libor is based largely hypothetical. In fact, in his speech announcing that Libor would cease, FCA chief executive Andrew Bailey said that for one of Libor's 35 currency and maturity iterations, the dozen banks that submit quotes had, in all of 2016, done just 15 transactions big enough to be relevant.

The FCA sounded Libor's death knell as regulators were still scrambling to settle on replacement rates. In June, the Alternative Reference Rate Committee (ARRC), a group convened by the Federal Reserve, recommended that the U.S. shift to using the Treasury repo rate—what people pay to borrow money overnight using U.S. Treasuries as collateral. The Fed has put out a request for comments on the recommendation and says it will begin publishing the new rate sometime next year.