A secretive SAS unit has been put on standby to “extract” UK officials from Ukraine if war breaks out, the Daily Mirror can reveal.
Members of SAS E Squadron will lead a rescue if Russian troops and armour pour deep into Ukraine.
As many as 126,000 Russian combat forces are now amassed close to Ukraine’s border - including ground troops and amphibious attack personnel.
More are pouring into neighbouring Belarus, putting Ukraine’s capital Kiev at risk if President Vladimir Putin orders a full-scale invasion.
Moscow’s commanders have ordered large amounts of multi-launch rocket systems to be transported into Belarus, which could be used as a launch point for an invasion.
American President Joe Biden has already warned he believes an attack is “imminent” and bungled by hinting an American response depends on the severity of an incursion.
Many observers believe full-scale invasion is unlikely but if that takes place British Foreign Office officials may be rescued by special forces.
It is believed crack UK special forces may also be placed strategically within Ukraine acting as military advisers, along with US counterparts.
One security source told the Daily Mirror: “British intelligence is watching developments all over Ukraine but in particular in Crimea, which is occupied by Russian forces.
“All of the other manoeuvres may have been a ruse to deflect from Crimea and Putin may want to expand it towards contested Donbas.”
Tonight the UK Ministry of Defence told the Daily Mirror: "We do not comment on special forces."
Ukraine’s troops along a 250-mile trench border through Donbas, in Donestk and Luhansk, are on high alert for an attack by Moscow’s forces.
They are preparing to launch a guerrilla warfare against attacking Russian soldiers if they roll over the frontline, using UK-supplied shoulder-launched NLAW missiles.
They have pitted crossing points with anti-tank mines and other defences, preparing to retreat and then launch counter-attacks against an invasion force.
Experts warned Russia will act quickly and decisively if it invades.
'Russia to act quickly and decisively in invasion of Ukraine'
Hugo Crosthwaite, lead analyst for Eurasia at security intelligence firm Dragonfly, said the Russian plan was to get concessions on Ukrainian NATO membership and Donbas.
Putin is keen to stop Ukraine from becoming a NATO member but the US has insisted on making no promise.
Moscow also wants pro-Russian separatist occupied Donbas to be officially part of Russia and some experts believe sanction threats will not deter the Kremlin.
He said: “A Russian invasion of Ukraine is a likely scenario in the coming weeks.
“There have been several military and diplomatic developments that point to a sustained severe interstate conflict risk, including Russian troops deploying to Belarus and further efforts by the Kremlin to create a pretext for an attack.
“The combined efforts of the US, the EU and NATO to deter Russia in recent months appear to have failed.
“Even the sanctions the US has threatened seem to have had little impact, with Russia having effectively sanction-proofed its economy.
“Russia’s military build-up means it can now begin a ground operation or launch a devastating missile and artillery attack at little to no notice.
“Rhetoric from the Kremlin has become more belligerent, including unevidenced accusations that the US is training ‘anti-Russia’ units and Ukraine is preparing to attack separatist forces in Donbas - a tactic the Kremlin used in 2014 to justify its annexation of Crimea.
“We still doubt that Russian forces would attack Kyiv, either via ground assault or air and missiles, and assess that Russia’s main goal in any military operation would be to force Ukrainian President Volodymyr Zelensky to resign quickly, and offer wide-ranging concessions from NATO membership to the status of Donbas.”
