The UK government on Wednesday raised tensions with the Kremlin by announcing it would expel a Russian defence attache for being "an undeclared military intelligence officer".
Interior minister James Cleverly told parliament the UK would also remove the diplomatic status of several Russian-owned properties, including one in Sussex, southern England, and another in London "which we believe have been used for intelligence purposes".
There would also be new restrictions on Russian diplomatic visas such as a cap on the length of time Russian diplomats can spend in the UK, he added.
Moscow hit back, calling the allegations "outright lies".
The UK's move comes as it is concerned at an apparent increase in "malign" Russian activity on British soil, including an arson attack on a Ukrainian-linked business allegedly orchestrated by the Kremlin.
A British man who it is claimed has links to the Wagner Group was charged in connection with that case last month.
London has previously accused Moscow of being behind the poisoning of a Russian former double agent and his daughter in Salisbury in southwestern England, and of a spate of cyberattacks and disinformation campaigns.
The UK is currently a staunch NATO backer of Ukraine, providing training for troops and military equipment in the fightback against Russia.
Cleverly said the new package of measures was intended "to make clear to Russia that we will not tolerate such apparent escalations".
He warned that Moscow would make accusations of Russophobia and spread conspiracy theories in response to his announcement.
"This is not new and the British people and the British government will not fall for it, and will not be taken for fools by (President Vladimir) Putin's bots, trolls and lackeys," Cleverly said.
"Russia's explanation was totally inadequate. Our response will be resolute and firm.
"Our message to Russia is clear: stop this illegal war, withdraw your troops from Ukraine, cease this malign activity."
The UK had summoned the Russian ambassador to inform them of the measures and "reiterate that Russia's actions will not be tolerated".
The defence attache, Maxim Elovik, is thought to have been in the UK since at least 2014 and has been pictured at wreath-laying ceremonies commemorating Russian war dead in 2020 and 2023, British media reported.
Before his posting in London, he worked at Russia's embassy in Washington.
Other "malign activity" believed to be linked to Russia involved five Bulgarian nationals who have been charged with conspiring to commit espionage in the UK on behalf of Russia.
"London has decided to use outright lies to justify its anti-Russian measures," foreign ministry spokeswoman Maria Zakharova said. "Our response will be tough and calibrated," she added, without elaborating.
Russia's Federal Security Service (FSB) has also been linked to a number of cyber activity incidents in the UK such as targeting British parliamentarians through phishing campaigns, hacking and the leaking of UK-US trade documents, a government statement said.
There had also been interference against a UK think-tank working on defending democracy against disinformation, it added.
The 2018 Salisbury poisoning attack on ex-double agent Sergei Skripal and his daughter Yulia led the UK and its European allies to act to make it harder for Russian intelligence services to operate.
Measures included the expulsion of 23 undeclared Russian intelligence officers from the UK.
