A former security guard who spied for Russia while working at the British embassy was "paid for treachery" and had an "ongoing relationship" with Moscow, a judge in London said on Thursday.
David Ballantyne Smith, 58, originally from Scotland, admitted eight breaches of the Official Secrets Act.
At the Old Bailey, he was convicted of spying activities between 2018 and 2021.
Justice Mark Wall said he faces a jail term of up to 14 years. He said Smith had passed sensitive materials to the Russian embassy in Germany, and was "paid for treachery" by Russia.
Smith claimed he had passed information to Russia twice to "cause embarrassment" to the UK.
Mr Justice Wall said he was "sure that ... in 2018 and 2019, the defendant was collecting information from the embassy with a view to passing it on at some stage".
The judge said Smith "was motivated by his antipathy towards this country and intended to damage this country's interests by acting as he did".
He concluded Smith had told a string of lies in his evidence and was, in fact, a “dedicated” spy on the Russian payroll.
Smith, a former RAF serviceman, was caught in a sting operation. An MI5 representative, identified in court only as 2093, placed Smith's spying in the context of relations with Russia at the time.
The officer said: "At the time the UK was engaged in the calling out of various activity undertaken by Russia, including significant concerns at the amassing of military personnel and the activity taking place on the border of Ukraine."
A security review had to be carried out for every member of embassy staff.
Neil Keeping, of the National Crime Agency, told of the potential "severe" consequences had sensitive details written on a whiteboard and filmed by Smith become known.
He said there were "potentially catastrophic" consequences for disclosure of staff details linked to their "key numbers" and addresses.
He said: "It put at risk each and every UK officer based in Berlin from any kind of attack.
"The consequences of that document being provided were potentially catastrophic."
The embassy's head of security Bharat Joshi estimated the cost of security measures to the taxpayer at £820,000 ($921,670).
He said: "They had to inform the individuals identified in this material that they must work on the assumption their personal details had been passed on and the staff have had to manage that.
"This had a significant and negative impact on many embassy staff, as described, feelings of anger, betrayal and upset and concern at the implications of their details being shared with a hostile state actor."
Smith will be sentenced at London’s Old Bailey court on Friday.