Boris Johnson will fly out to Poland and Estonia for crisis talks over the escalating conflict in Ukraine.
Downing Street confirmed the Prime Minister would visit the region on Tuesday for meetings with allies and NATO Secretary-General Jens Stoltenberg.
The Russian offensive in Ukraine has entered its fifth day, with Vladimir Putin's forces targeting the capital Kyiv and major cities of Kharkiv and Chernihiv.
Defence Secretary Ben Wallace warned the invasion was likely to become "more violent" as Russia resorts to more brutal tactics to advance.
He said the Russian handbook pointed to "ruthless, indiscriminate bombing of cities" with "high casualty levels" as Moscow's troops meet stubborn resistance.
British Government insiders believe Mr Putin expected to sweep through the country and Russian forces have been caught off guard by the level of opposition from Ukrainian people.
He is also believed to have overestimated Russia's military capacity, with reports of some units struggling with fuel shortages.
There is growing hope in Government that sanctions slapped on Russian banks and oligarchs are starting to have an impact.
Downing Street accused Mr Putin of using a "distraction technique" for his stalling campaign - by trying to blame Foreign Secretary Liz Truss for his own nuclear threat.
Mr Johnson told Cabinet on Monday that Mr Putin had made a "colossal mistake" believing his troops would be "garlanded with roses" by Ukrainians.
The Prime Minister's official spokesman said: "The Prime Minister said it is becoming clearer with each day that Putin had made a colossal mistake believing that the guns of his tanks would be garlanded with roses when instead the Ukrainian people had put up a fierce resistance in defence of their homeland.
"He said latest intelligence suggested Putin's advances had been hampered by logistical problems and the heroic efforts of the Ukrainian military who are inflicting significant casualties on Russian troops."
The spokesman added: "He [Mr Johnson] said Putin had also underestimated Western unity and the strength of the sanctions his action could lead to.
"The Prime Minister said Putin must fail in his attempts to subjugate Ukraine and the UK would continue its efforts in three main areas to achieve this: economically, diplomatically and militarily."
It comes as Home Secretary Priti Patel was expected to set out more details on help for Ukrainian refugees fleeing the Russian invasion.
The Government announced the relaxation of visa rules for immediate family members of Ukrainians settled in the UK after coming under intense criticism over the weekend.
But Labour demanded ministers extend it to all relatives and to provide a "broader sanctuary route" to help other Ukrainians.
The PM's spokesman said: "We will be doing even more with the Home Secretary expected to say a little more in the House later today."
Downing Street also contradicted Ms Truss for saying she supported people who wanted to travel out to Ukraine to join the fight.
The PM's spokesman said: "We fully recognise the strength of feeling about British people wanting to support the Ukrainians following the Russian invasion.
"There's advice up on travelling to Ukraine, we currently advise against travel to Ukraine."
