Boris Johnson jets to Germany tomorrow for a crunch security conference amid mounting fears Russia will invade Ukraine.
The Prime Minister is expected to give a speech at the annual Munich Security Conference as tensions grip Europe.
Whitehall figures are now said to be convinced Vladimir Putin - who will watch nuclear missile tests tomorrow - will order Kremlin forces to attack.
Yesterday Boris Johnson, the US and NATO all accused Russia of staging “false flag” attacks as a pretext for invading Ukraine after a nursery was attacked.
UK Security Minister Damian Hinds said while an invasion was “not inevitable”, it could happen at “any time”.
“It might be imminent or it might take a little bit longer. We’re dealing with a regime here in President Putin who may pick his moment,” he told Sky News.
Mr Hinds added: "It is the position of the British Government that we have to be ready for whatever Vladimir Putin decides to do.
“The unavoidable truth is that there is this huge build-up of military personnel and equipment on multiple sides, multiple fronts of Ukraine.
“There is no sign of that build-up slowing, let alone reversing, so we need to be ready. But there is still a diplomatic route and that is what we encourage President Putin to take."
A Whitehall source told The Times: “He's going to do it, and it's going to be horrendous.”
Before flying to Munich, Boris Johnson will take part in a virtual meeting hosted by US President Joe Biden with the leaders of France, Germany, Italy, Poland, Romania, Canada, the EU and NATO.
A White House official said: “The President will speak with Transatlantic leaders on a phone call about Russia’s buildup of military troops on the border of Ukraine and our continued efforts to pursue deterrence and diplomacy.”
Foreign Secretary Liz Truss and Defence Secretary Ben Wallace are scheduled to appear on panels at the conference.
Mr Hinds told Sky News: “There is still a way out. It’s still with Russia. There is a diplomatic route and we strongly, strongly encourage Russia to take that route.”
A UK government source told Sky News: “President Putin needs to know the world is watching, and that we will hold him to account.
"We will also counter the disinformation he will inevitably seek to spread so that facts are known and the world knows and understands exactly what he’s up to”.
A diplomatic source told Reuters clashes that took place in eastern Ukraine on Thursday were the biggest since 2015.
It is part of a long-running conflict between Russian-backed separatists and the Ukrainian army.
The source said observers from the Organisation for Security and Cooperation in Europe (OSCE) had already recorded 80 ceasefire violations along the dividing line between the two sides as of Friday morning.
The Kremlin said today that Moscow was alarmed by the situation in eastern Ukraine's Donbass region and it looked potentially very dangerous.
But the UK, US and NATO have blamed Moscow for the clashes and said Russia is trying to orchestrate them as an excuse to start a war.
The UK Government is prioritising British nationals and their families leaving Ukraine as people flee the threat of war.
Asked about whether the Government had a plan to help Ukrainian refugees, Home Office minister Damian Hinds said: "First of all we hope the war doesn't take place, doesn't have to take place, that is with Vladimir Putin. It is very clear there is a diplomatic route available.
"I am very much focussed that British nationals who are in Ukraine and their immediate families, husbands, wives, their children, they are able to leave if that is what they wish to do."
