A merchant selling T-shirts promoting Donald Trump. A public education organization favoring Democratic candidates. A company in the firearms industry looking for assistance on an initial public offering.

All were businesses that were denied services by some of America’s largest financial institutions, including JPMorgan Chase and Bank of America, a Trump-appointed regulator said Wednesday.

In a 45-page report, the Office of the Comptroller of the Currency, or the OCC, detailed specific examples it found showing banks steering away from companies that they viewed as operating in politically sensitive areas. The banking regulator’s findings come after a yearlong investigation, sparked by President Trump’s claims that the largest banks in the country engaged in what the administration has termed “politicized or unlawful debanking activities.”“Today’s report provides irrefutable evidence that the nation’s largest banks have denied, debanked and discriminated against customers and lawful businesses,” Comptroller of the Currency Jonathan Gould said.

The report intensifies a political storm for banks during Trump’s second term. Banks had initially brushed aside claims of bias, saying they don’t close accounts for political or religious reasons. They have said decisions to avoid certain industries or clients are in accord with laws that make banks watchdogs for criminal activity and money laundering or are in response to other regulatory pressures meant to safeguard the banks from what is known as reputational risk.

But Trump officials have continued to dig into the matter. Now, regulators could seek to levy financial penalties against banks for debanking certain customers, even though the legal basis for doing so remains unclear.

The White House last year issued an executive order accusing banks of discriminating against conservatives and cryptocurrency companies and threatening fines for lenders that dropped customers for political reasons.

Trump has said that he and his businesses were debanked after his first term in office and the Jan. 6, 2021, riots at the Capitol led by a mob of his supporters. He and his family have sued JPMorgan and Capital One for closing their accounts.

The U.S. attorney’s office in D.C. has subpoenaed banks including Bank of America and JPMorgan and is assessing whether they violated a law passed after the savings and loan crisis of the 1980s. The OCC on Wednesday said it was considering its own actions to prevent further discrimination.

Democrats including Sen. Elizabeth Warren of Massachusetts have also alleged the banks cut off clients without valid reasons, though they have focused on access to banking services by low-income Americans, immigrants and people with criminal convictions.

The OCC on Wednesday pinpointed reputational risk and bank’s environmental, social and corporate-governance policies as two of the primary drivers of the account closures, which it said affected businesses connected to firearms, private-prisons, cryptocurrencies and abortion. It said banks largely lacked clear standards for what constituted reputational risk.

Regulators had viewed reputational risk as an issue that could impact a bank’s financial stability but Trump’s regulators have moved to eliminate it as a factor in bank supervision.

The report released Wednesday offers examples of debanking by each of the nine largest banks that the OCC had said it was examining as part of its review, including quotes and emails from staff and compliance officials.

In one example, JPMorgan repeatedly denied services to a politically leaning retailer between 2021 and 2023, the OCC said in its report.

“As more far-right competitor [companies] step in to take this company’s extreme clients, it may become more mainstream and more appropriate for JPM to bank,” the report quoted bank documents saying.

The report listed several examples involving a payments processing subsidiary JPMorgan acquired, WePay. In September 2021, WePay closed an account for a merchant it said violated its terms of service for selling merchandise that raised “red flags,” the report said. The merchandise included: “Trump-Won-3-x-5-foot-flag-with-grommets,” “Biden-Illegitimate-Tshirt,” and “The-Big-Lie-Sticker.”

JPMorgan said its policies “prohibit discrimination against customers based on political or religious views.”

“A process used by WePay, a small technology company we acquired years ago and have since wound down, did not meet our expectations,” the bank said. “We identified it, corrected it, and the business no longer operates.”

In another instance, TD Bank decided against extending a Covid-19 era emergency loan to a nonprofit promoting anti-immigration positions. The bank’s decision was influenced in part by the Southern Poverty Law Center, which had designated the nonprofit a “hate group.”

Capital One, the report said, rejected an organization supporting reproductive rights, including abortion access. One risk employee pointed out that its “[c]ontributions to politicians are about 95% Democrat.”The OCC noted that, in contrast, the big banks were “often finding ways to retain highly lucrative, existing bank clients” in wealth management even if they had public issues.

Write to Dylan Tokar at dylan.tokar@wsj.com