Washington (CNN) US Ambassador to the European Union Gordon Sondland's cell phone call to President Donald Trump from a restaurant in Ukraine this summer appears to be a shocking security breach that raises significant counterintelligence concerns, according to several former officials, who told CNN there is a high probability that intelligence agencies from numerous foreign countries, including Russia, were listening in on the conversation.

"If true, the cell phone call between Ambassador Sondland and President Trump is an egregious violation of traditional counterintelligence practices that all national security officials -- to include political appointee ambassadors such as Sondland -- are repeatedly made aware of," according to Marc Polymeropoulos, a former CIA officer who oversaw operations in Europe and Russia before retiring this summer.

"I cannot remember in my career any time where an ambassador in a high counterintelligence environment like Kiev would have such an unsecure conversation with a sitting president. This just should not happen," he said.

Bill Taylor, the top US diplomat in Ukraine, revealed during the first public impeachment hearing Wednesday that a member of his staff, who was accompanying Sondland to meetings in Kiev, saw the ambassador call Trump from his cell phone and overheard the President asking about "the investigations."

Taylor confirmed that he had come to understand the term "investigations" meant matters related to the 2016 election and to probes of Joe and Hunter Biden and Burisma

"Ambassador Sondland told President Trump that the Ukrainians were ready to move forward," Taylor told lawmakers.

The call occurred on July 26, according to Taylor -- the day after Trump's phone call with Ukrainian President Volodymyr Zelensky that prompted a whistleblower complaint alleging Trump solicited "interference" from a foreign country to help his 2020 presidential campaign.

This new information could strengthen Democrats' argument for impeachment that Trump engaged in an alleged quid pro quo but it also serves as another example of top US officials ignoring security protocols related to sensitive communications.

It remains unclear if Sondland's cell phone was encrypted but US ambassadors do not typically have that type of protection on their mobile devices, according to current and former US government officials.

The State Department did not immediately respond to CNN's request for comment on whether Sondland's cell phone was outfitted with any sort of enhanced security.

Normally, a US ambassador talking to the President would do so from the embassy using a secure line, one former intelligence official told CNN. "Of all the communications, cell phones are even more vulnerable than non-secure landlines, which are way more vulnerable than secure communications facilities," the former official said.

That lapse was only amplified by the fact that Sondland made the call in public, where it could have been easily overheard and in a foreign country that is already being targeted by foreign adversaries of the US, including Russia, current and former officials said.

'Crazy for today's age'

"Why a president is talking to an ambassador on a non-encrypted telephone is crazy for today's age, and worse in public," said Todd Carroll, a former FBI official who served as assistant special agent in charge of the cyber and counterintelligence branch.

"Ukraine is one of the most open areas for intelligence agencies to work in. Both sides. I was told when I was there in 2010 that expect all your calls to be monitored," Carroll added.

The Russians, in particular, maintain a particularly large intelligence presence in Ukraine and are known to target the communications of US officials.

"There is little doubt that the Russians and perhaps multiple other foreign intelligence services would have intercepted this call. Moscow undoubtedly would have been pleased," according to Polymeropoulos.

"This would offer the Russians some important validation that President Trump was in effect doing exactly what Moscow almost certainly was already aware of: that our President was inserting a serious wedge into ongoing US security assistance programs that Ukraine so desperately needed in their ongoing battle with Russia," he added.

Fiona Hill, a former Russia aide on Trump's National Security Council, testified in October that she had previously tried to get Sondland to stop using his personal cell phone for work.

"I mean, some of it was comical, but it was also, for me and for others, deeply concerning. And I actually went to our Intelligence Bureau and asked to have (redacted) sit down with him and explain that this was a counterintelligence risk, particularly giving out our personal phone numbers," Hill told House investigators.

"All of those communications could have been exfiltrated by the Russians very easily," she said.

CNN has previously reported that Trump was using his personal cell phone to contact outside advisers, raising concerns that his calls were vulnerable to eavesdropping from foreign governments.

"All communications devices of all senior government officials are targeted by foreign governments. This is not new," Bryan Cunningham, executive director of the Cybersecurity Policy and Research Institute at the University of California-Irvine, told CNN last year.

"What is new in the cell phone age is the ease of intercepting them," Cunningham added. "Of course, calls are only secure if both parties use a secure device."