Washington (CNN) President Donald Trump has continued to use his personal cell phone to make calls, despite repeated warnings from his staff that the practice could leave him vulnerable to foreign surveillance, multiple officials told CNN.

The Democratic impeachment inquiry has resurrected concerns about the security and potential vulnerability of the President's communications. Witness testimony revealed some top officials repeatedly failed to follow protocol intended to prevent sensitive phone conversations, including those involving the President, from being intercepted by foreign intelligence services.

Several former US officials have told CNN it is highly likely that US Ambassador to the European Union Gordon Sondland's cell phone call to Trump from a restaurant in Ukraine over the summer was picked up by intelligence agencies from numerous foreign countries, including Russia.

Normally, a US ambassador talking to the President would do so from the embassy using a secure line, one former intelligence official told CNN. Cell phones, the former official said, were much more vulnerable than calls made at secure communications facilities.

The 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.

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