Andy Burnham has been warned that Britain faces Russian retaliation over its support for Ukraine.
Security experts have told the Prime Minister to be prepared for a 'direct response' from Vladimir Putin. The Kremlin is set to target the UK, sources said, after miscalculating Mr Burnham's commitment to Kyiv.
Putin had been advised the new incumbent in No10 would be 'softer than [Sir Keir] Starmer'.
But the Russian president's hopes were dashed by the Prime Minister's moves to fortify the UK-Ukraine military alliance in recent weeks, swapping missile blueprints and battlefield AI with Moscow's enemy.
Putin is also believed to be coming under pressure within Russia to challenge Britain's increasing involvement in the conflict.
The shock alert for the PM came as the head of the US's Central Intelligence Agency (CIA) made an emergency visit to Moscow yesterday.
It was reported that spy chief John Ratcliffe issued a warning to the Kremlin not to escalate the conflict in Ukraine, either by using a nuclear weapon or targeting Kyiv's Western allies.
The 'just don't do it' advice was based on US intelligence that Russia is primed to act immediately and dramatically after its campaign on the battlefield stalled again.
Andy Burnham has been warned that Britain faces Russian retaliation over its support for Ukraine. He is pictured with President Volodymyr Zelensky on Monday
Mr Burnham reacts is shown the rubble and wreckage left on the buildings and site of a strike near the Lukanivska market in Kyiv
UK security chiefs were on Tuesday night primed for a Kremlin attack on Britain – either towards Mr Burnham or by targeting defence sites.
A senior source told the Daily Mail: 'The question being asked is, "How long do we keep poking the bear before it bites?"
DAN HODGES: What my senior security sources are telling me about Russia's alarming plans for Britain
'An attack on the UK's defence and security infrastructure is inevitable, based on our high-profile support for Ukraine.
'Ourselves and the French are front and centre, and Burnham's visit to Kyiv and his forthright stance will have been noticed at the Kremlin.
'There had been some optimism in Moscow that he would be softer than Starmer on Ukraine.
'That was not the case. Russia will want to make a statement, through actions rather than words. That could be a drone attack, or an arson attack. There are Russian sleeper cells in this country that could do either.'
The source added: 'Russia is testing our security infrastructure on a daily basis... Something bigger is only a matter of time. Russia just needs to keep it low enough to avoid a military response by Nato.'
Vladimir Putin, pictured in July, is believed to be coming under pressure within Russia to challenge Britain's increasing involvement in the conflict
In June, UK forces boarded a Russian shadow fleet vessel for the first time
In this photo taken from video distributed by Russian Defense Ministry Press Service in 2025 ans is said to show a Russian soldier launching a drone in an undisclosed location in Ukraine
Other sources said that Russia's sudden withdrawal of its ambassador to London was being taken as a sign that 'something big is coming'.
Major UK defence manufacturers participating in weapons production are stepping up their physical security and adopting measures to prevent cyber attacks.
A Kremlin adviser brazenly suggested factories producing drones for Ukraine could be targeted by 'unknown sources'.
Former Russian deputy foreign minister Andrei Fedorov said he could not '100 per cent exclude' the prospect of 'semi-military' action against Britain following Mr Burnham's visit to Kyiv on Monday.
During the trip, which was the Prime Minister's first overseas since entering No10, the UK reiterated its 'unwavering' support for Ukraine, which in turn handed over what officials have called a 'goldmine' of battlefield data to help bolster British security.
Mr Fedorov said that 'sooner or later, there might be not only verbal but maybe some visible reaction from Russia towards the UK'.
Asked to elaborate on what this might look like, he said: 'It might be so-called technical reaction, maybe some hackers attack from [an] unknown source.
'There might be different political steps and I cannot exclude 100 per cent that there might be some semi-military [action].'
Pressed to define 'semi-military' he told the BBC's Newsnight: 'For example, something might happen with enterprises, with the factories which are producing drones for Ukraine.'
Asked whether this meant factories in Britain being attacked militarily, the Kremlin adviser said: 'They could be attacked, not by Russia, but by... from unknown sources.'
In April, a Russian SU-35 fighter flew within 20ft of a RAF Rivet Joint intelligence-gathering plane
The vulnerability of Britain's energy infrastructure to online attack by either Russia, Iran or China has been exposed recently.
A hydroelectric plant was shut down for four days last month after being targeted by Iranian hackers.
Russia has become more brazen in recent months, with its jets flying close to British military aircraft over the Black Sea.
In April, a Russian SU-35 fighter flew within 20ft of a RAF Rivet Joint intelligence-gathering plane.
The Ministry of Defence condemned the act as 'dangerous and unacceptable'.
In June, UK forces boarded a Russian shadow fleet vessel for the first time. Royal Marine commandos landed on the Smyrtos oil tanker in the English Channel, in a major blow to Russia's war chest.
Russia is preparing to mobilise 300,000 troops to fight in Ukraine, where it has sustained 1.5million casualties since invading in 2022.