A U.S. ambassador’s cellphone call to President Trump from a restaurant in the capital of Ukraine this summer was a stunning breach of security, exposing the conversation to surveillance by foreign intelligence services, including Russia’s, former U.S. officials said.

The call — in which Trump’s remarks were overheard by a U.S. Embassy staffer in Kyiv — was disclosed Wednesday by the acting U.S. ambassador to Ukraine, William B. Taylor Jr., on the dramatic opening day of public impeachment hearings into alleged abuse of power by the president.

“The member of my staff could hear President Trump on the phone” asking U.S. Ambassador to the European Union Gordon Sondland about “the investigations,” Taylor testified, referring to the president’s desire for a probe of the son of Trump’s potential political opponent in 2020, Joe Biden, and the Ukrainian energy company on whose board Hunter Biden once served.

Sondland, Taylor said, told Trump in that conversation that “the Ukrainians were ready to move forward” on the investigations.

The U.S. Embassy staffer who overheard the call, political counselor David Holmes, is scheduled to testify Friday before House impeachment investigators in a closed session.

“The security ramifications are insane — using an open cellphone to communicate with the president of the United States,” said Larry Pfeiffer, a former senior director of the White House Situation Room and a former chief of staff to the CIA director. “In a country that is so wired with Russian intelligence, you can almost take it to the bank that the Russians were listening in on the call.”

The impromptu Sondland-Trump conversation took place a day after an official call July 25 between Trump and Ukrainian President Volodymyr Zelensky, in which Trump had pressed for investigations. Zelensky at that time was eager to secure a White House visit. The Trump-Zelensky call and the pressure on the authorities in Kyiv to conduct investigations that could aid Trump’s 2020 campaign are at the heart of the impeachment hearings.

It was also noteworthy in that ambassadors typically don’t just pick up the phone and call presidents. “They never do so to discuss Ukraine policy,” former U.S. ambassador to Russia Michael McFaul said in a tweet. “Doing so on a cellphone from Kyiv means [the] whole world was listening in.”

Russia already has shown its ability to monitor U.S. diplomats’ calls in Kyiv, and the Kremlin has no hesi­ta­tion in leaking them when it suits its interests. In early 2014, an intercepted phone conversation between then-Assistant Secretary of State for European and Eurasian Affairs Victoria Nuland and then-U.S. Ambassador to Ukraine Geoffrey Pyatt appeared on YouTube. On the call, Nuland was heard referring dismissively to slow-moving European Union efforts to address a looming political and economic crisis in Ukraine. “F--- the E.U.,’’ she said.

She also was heard assessing the political skills of Ukrainian opposition figures with unusual candor. The leaked call was embarrassing to Washington and appeared to be an effort by Moscow to drive a wedge between the United States and the European Union. The State Department acknowledged the call was authentic, and Nuland apologized to E.U. officials. But U.S. officials also were quick to blame Russia for the leak, describing it as a form of information warfare.

Calling a president from a cellphone violates protocols set up to protect senior administration officials’ communications. “It’s indicative of a lack of concern for operational security,” said a former senior U.S. intelligence official, who spoke on the condition of anonymity to avoid being accused of making statements motivated by political bias. Senior officials, he said, “are routinely briefed on the threats to their communications. You could assume that talking on an unencrypted line from a foreign country would be on that list.”

It is also dangerous for a president to take an off-the-books call like that, Pfeiffer said. That is why call logs are kept, he said. Without them, someone could assert that the president said something on a call, and a log “protects the president’s ability to deny something happened,” he said. “Good bureaucratic record-keeping is a protection for someone in the position of the president.”

This is not the first time questions have arisen over Trump’s un­or­tho­dox phone use. He has been known to give his personal cellphone number to other world leaders, despite aides’ warnings that his cellphone calls are not secure. Russia and China in particular have targeted his personal cellphone calls, the New York Times reported.

In general, Trump has appeared unusually cavalier about operational security. In February 2017, at his Mar-a-Lago Club in Palm Beach, on an open terrace in full view of other diners, Trump and Japan Prime Minister Shinzo Abe had a sensitive discussion about how to respond to a ballistic missile test by North Korea.

Greg Miller and Karoun Demirjian contributed to this report.

CORRECTION: An earlier version of this story misspelled Ukrainian President Volodymyr Zelensky’s first name. The story has been updated.