ussian forces are massed able to launch a lightning attack to try to seize the Ukrainian capital Kiev within days.
Special forces would be expected to be part of a Russian invasion which could swiftly encircle Ukraine’s best-trained troops in the east of the country, cutting them off from defending Kiev.
A rapid capture of Kiev, after potential major bombing of key sites in the city, would severely damage the country’s political and military decision-making capabilities, and potentially pave the way for a pro-Russian administration to be put in place.
But an invasion also risks leading to a large number of Ukrainian and Russian casualties in a conflict which could last years.
Some Russian troops in Belarus are understood to have already moved closer to the border with Ukraine.
Other units in Russia are believed to have left centralised areas to move closer into the field which puts them in a better position to advance into Ukraine.
Russian military chiefs are expected to try to avoid street-by-street fighting in the Ukrainian capital given that there could be many civilian, as well as military, casualties.
However, previous military operations suggest they would order such urban incursions if they thought it necessary to seize the city, even if it meant appalling bloodshed.
Boris Johnson warned on Monday that the situation in Ukraine is “very, very dangerous” with Britain fearing that a Russian invasion is now “very imminent”.
He urged Vladimir Putin to step back from “the edge of a precipice”, having deployed a force around Ukraine which is far bigger than that used to seize Crimea in 2014 and is believed to be the largest military build-up since the end of the Gulf War.
The Russian president has denied he has any plans to invade.
However, the sheer scale of the Russian force, with more than 130,000 troops, and its make-up suggests it has been put together for an invasion, even if the decision has not yet been taken by Mr Putin to order it.
Around 100 battalion-tactical groups are thought to have already been deployed as part of the huge build-up and at least 14 more to be on the move, which in total represents over 60 per cent of Russians ground fire power.
Short-range ballistic missiles have been deployed in Russia and Belarus and the Kremlin has sent more military vessels to the Black Sea, including landing craft.
Britain’s armed forces minister James Heappey stressed on Monday morning: “Perhaps more ominously, although it grabs less headlines, all of the combat enablers, the logistics, the fuel, the medical supplies, the bridging assets are all also now in place.”
Russian officers are thought to be working on the basis that they are preparing for an invasion rather than a military exercise.
The key decisions could be taken within days, possibly even for an advance on Wednesday.
Attacks could take place from Belarus in the north, Russia to the east or Crimea to the south, though, they may not all be ordered at the same time.
The Kremlin may launch a “false flag” operation as a justification to send troops into Ukraine to defend pro-Russian enclaves, or in apparent retaliation to alleged Ukrainian aggression.
Mr Johnson and US president Joe Biden were holding crisis talks on Monday afternoon.
Downing Street said the Prime Minister was being briefed by security chiefs during the day on the threat of an attack on Ukraine.
The Prime Minister’s official spokesman said an invasion this week was a “grave possibility”, even with the warning of damaging sanctions on Moscow including to its financial sector.
“There remains a window of opportunity to de-escalate and pursue a diplomatic path,” he said.
But he stressed: “There are no signs as we speak of de-escalation.”
Given the scale of the military deployment, some experts have questioned whether Mr Putin could now withdraw his forces without losing face.
However, the Russian president could argue that he had made the West open up talks about European security, force and missile deployment, and about transparency of actions.
However, if he does order an invasion, his big gamble would be partly how many Ukrainians would welcome Russian soldiers.
Some Ukrainians would but others would not and the Kremlin may have underestimated how many would oppose it, which could risk a bloody quagmire war with Russian troops bogged down in Ukraine for years at huge economic and human cost.