Capital One has been accused of defrauding customers out of $2 billion

Capital One’s ad had many of the same promises other banks offer for high-yield savings accounts: There were no fees, no minimum deposits, and the money would earn much more in interest than traditional bank accounts.

“What’s the catch? It’s not there,” boasted the bank.

But there was one, and it cost savers more than $2 billion, according to a lawsuit filed Tuesday against Capital One by the Consumer Financial Protection Bureau.

For years, Capital One kept interest rates on the high-yield product artificially low last summer, at 0.30 percent a year, for example, even after the Federal Reserve raised rates above 5 percent.

Banks are generally allowed to pay as little interest as their customers allow, but Capital One went too far, according to the lawsuit, by intentionally confusing its customers about its products. The bank operated two separate, nearly identical account options — 360 Savings and 360 Performance Savings — and prohibited its employees from voluntarily disclosing information about 360 Performance Savings, the higher-paying one, or promoting it to existing customers.

In its lawsuit, the consumer bureau called the practice deceptive, abusive and illegal. “The only thing that has ever significantly differentiated 360 Performance Savings from 360 Savings is the former product’s higher interest rate,” the lawsuit states.

Capital One spokesman Sie Soheili declined to comment on the specific allegations in the lawsuit, but said the bank, the nation’s ninth largest, disagrees with the allegations and will challenge them in court. The bank is trying to complete its takeover of credit card issuer Discover.

The lawsuit is the latest in a series of last-minute warnings from the consumer bureau before the inauguration of President-elect Donald J. Trump. Republicans largely loathe the agency, which was created in 2011 and which they say is an example of bureaucratic overreach, even though Mr. Trump kept it alive during his first term.

Almost every business day in the past month, the agency has filed a lawsuit or proposed rule change or other pending action against a major financial institution. Tuesday’s lawsuit was not the agency’s first in recent weeks involving Capital One; last month, the regulator accused Zelle, a money transfer app co-owned by the bank, of failing to protect its customers from more than $800 million in fraud.

Mr. Soheili said in a statement that regulators were continuing their “recent pattern of filing lawsuits at the 11th hour before a change in administration.”

A spokeswoman for the consumer bureau, Tia Elbaum, responded: “When we see violations of the law, we will take action.”