U.S. 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, CNN wrote.

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

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

"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 U.S. officials ignoring security protocols related to sensitive communications, CNN said.

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