Government authorities are planning to arrest two former JPMorgan Chase employees suspected of masking the size of a multibillion-dollar trading loss, a dramatic turn in a case that tarnished the reputation of the nation’s biggest bank and spotlighted the perils of Wall Street risk-taking.

The former employees, who worked in London, could be arrested in the coming days, according to people briefed on the matter. The action, the people said, would involve criminal fraud charges.

The employees — Javier Martin-Artajo, a manager who oversaw the trading strategy, and Julien Grout, a low-level trader in London — could ultimately be extradited under an agreement with British authorities. Yet the people briefed on the matter, who spoke on the condition of anonymity, cautioned that it is unclear whether British authorities will be able to locate the men, who are natives of other European countries.

Representatives for the F.B.I. and the United States attorney’s office in Manhattan declined to comment, as did a spokesman for JPMorgan. A lawyer for Mr. Martin-Artajo did not respond to an e-mail. A lawyer for Mr. Grout could not be located.

The plan to arrest the traders hints at an aggressive new stance from the government, which has come under fire for prosecuting only a few Wall Street employees tied to the 2008 financial crisis. Taking aim at employees of a Wall Street giant like JPMorgan, even when they fall below the executive ranks, could send a warning shot across the financial industry.

The losses at the heart of the JPMorgan case stemmed from outsize wagers made by the traders at the bank’s chief investment office in London. The traders used derivatives — complex financial contracts whose value is typically tied to an asset like corporate bonds — to bet on the health of large corporations like American Airlines.

Those trades soured last year, racking up steep losses for the bank. JPMorgan, which initially disclosed the problem last May, has since announced that the losses reached more than $6 billion.

After more than a year of gathering evidence about the losses, federal prosecutors and the F.B.I. in Manhattan have concluded that the two employees understated the value of their trades to hide the problem from executives in New York. Studying internal e-mails and phone recordings that shine a light on how the employees valued the trades, authorities came to believe that Mr. Martin-Artajo directed Mr. Grout to falsify internal records.

Those actions, JPMorgan told authorities, caused the bank to lowball the losses. Last July, the bank restated its first-quarter 2012 earnings downward by $459 million, conceding errors in the valuations.

The charges hinge on the cooperation of another JPMorgan trader, Bruno Iksil, nicknamed the London Whale because of his role in the unusually large bet. Despite initially personifying the trade — the blowup is referred to colloquially as “the London Whale” — some investigators concluded that he was unfairly blamed.

After giving multiple interviews to authorities, first at a meeting in Brussels and then New York, Mr. Iksil secured a cooperation agreement from the government. It is unclear, however, whether he will face separate charges.

While authorities are not pursuing charges against JPMorgan’s top executives, according to the people briefed on the matter, the bank is nonetheless bracing for civil charges from regulators. The Securities and Exchange Commission, which is expected to cite the bank for lax controls that allowed the traders to undervalue the bets, could strike a settlement with the bank as soon as this fall.

The Financial Conduct Authority, a British regulator, also plans to fine the bank in the coming months, one person said.

In an unusually aggressive move, the S.E.C. is seeking to extract an admission of wrongdoing from the nation’s biggest bank. If JPMorgan concedes to that demand, such an admission would reverse a longtime practice at the S.E.C., which has allowed defendants for decades to “neither admit nor deny wrongdoing.” The people briefed on the case added that the agency had not threatened to file civil charges against JPMorgan executives.

As the criminal case progresses, the government could face challenges in the courtroom. For one, Wall Street cases are unusually difficult to prove to jurors who must grapple with financial jargon.

That hurdle is particularly high in this case, which centers on the vagaries of rules that even seasoned Wall Street employees struggle to interpret. Under the rules, traders are granted some leeway to value their trades on derivatives contracts because actual prices may not be readily available, presenting a challenge to prosecutors who must prove that employees intentionally cloaked losses.

Even though JPMorgan itself is not the target of criminal scrutiny, the case casts an unwelcome spotlight on the bank. The case could highlight the way the traders breached the bank’s own risk limits to make their bets — suggesting that JPMorgan, once hailed for its risk management, had porous controls.

The case also comes at a time when JPMorgan, which recently reported record quarterly profits, is already grappling with an array of regulatory woes. The bank faces inquiries from at least eight federal agencies, a state regulator and two European nations. Adding to its headaches over the trading losses, authorities are investigating the bank in connection with its financial crisis-era mortgage business. The bank, for example, recently disclosed that federal prosecutors in California are investigating whether it sold troubled mortgage securities to investors before the crisis. Regulators are also examining flaws in the bank’s debt collection practices.

The investigation into the trading losses stems from early 2012. As their bet worsened, JPMorgan has said, the traders started to underestimate the losses.

Since announcing the losses, the bank has overhauled its controls and ousted the employees involved in the bet. The bank’s chief executive, Jamie Dimon, has apologized for the breakdown.

The bank also commissioned an internal investigation into the trades, ultimately turning over its findings to federal authorities and a Senate subcommittee examining the losses.

An ensuing subcommittee report concluded that Mr. Martin-Artajo’s team of traders stopped recording the value of their bet in a “middle range,” shifting to some of the most generous possible figures.

In one recorded phone call referred to in the report, the London Whale, Mr. Iksil, told a colleague that the bank’s estimated losses were “getting idiotic.” Mr. Iksil added that “I can’t keep this going” and that he did not know where his boss in London “wants to stop.”

Reflecting concerns about the estimates, Mr. Grout kept a spreadsheet that tracked the difference between his valuations and the midpoint. The documents, according to the subcommittee’s report, showed that his valuations underestimated the losses by more than $400 million.

Mr. Grout, the subcommittee said, told Mr. Iksil in a recorded conversation, “I am not marking at mids as per a previous conversation.”