The information, which related to the use of laptops on aircraft, is understood to have been passed to the Americans by an ally who had apparently chosen not to share it with Moscow.
It was marked "codeword classified", and was highly sensitive.
Mr Trump has now said he "had the absolute right" to tell the Russians "acts pertaining to terrorism and airline safety".
So what are the rules around confidential information, and does he really have the right?
