The Federal Security Service (FSB), main successor to the Soviet-era KGB, said it had discovered that second secretary Albertus Gerhardus Janse van Rensburg was "carrying out intelligence and subversive activities that threaten the security of the Russian Federation".
A spokesperson for the UK's foreign ministry called the accusations "completely unacceptable," accusing Russia of harassing its diplomats by "pumping out malicious and completely baseless accusations".
As it wages war in Ukraine, Russia has accused the UK of pursuing espionage aimed at sowing discord within Russia at a level unseen since the depths of the Cold War.
The UK, which supports Ukraine with money and weapons, sees Russia as its biggest immediate threat and accuses Russian intelligence of mounting major cyber attacks, killings and sabotage campaigns across the world.
Russian state television aired videos that it said showed Janse van Rensburg meeting economic contacts in Moscow restaurants, and cross-country skiing beside what state TV said was a special defence ministry building.
State TV said Janse van Rensburg was the 16th UK diplomat to be expelled from Moscow in two years.
It was not possible to contact the diplomat.
The FSB said he had attempted to "obtain sensitive information during informal meetings with Russian experts in the field of economics".
"To avoid negative consequences, including criminal liability, the FSB of Russia recommends that compatriots refrain from holding meetings with British diplomats," it added.
As the US under President Donald Trump seeks to reset ties with Russia and broker peace between Russia and Ukraine, the UK has been granted the status of Russia's public enemy number one.
On state television, "Perfidious Albion" is cast as scheming behind the scenes to undermine Russian interests around the world.
Foreign diplomats in Russia say intrusive surveillance and harassment are frequent, and that a guide known as "Moscow Rules," developed in Soviet times to guard against complacency, has been updated.
Russia has imposed Soviet-style restrictions on most UK diplomats, requiring them to give notice of any plans to travel beyond a 120-km radius.
