The Prime Minister says the UK plans on being part of any new NATO deployments if Russia invades Ukraine.
It comes after Russia has massed an estimated 100,000 troops near Ukraine's border.
Boris Johnson told the House of Commons the Government cannot "bargain away" the vision of a free Europe which emerged between 1989 and 1991, the latter year being when the Cold War is generally considered to have ended with the collapse of the Soviet Union.
Mr Johnson told MPs: "The British Army leads the NATO battlegroup in Estonia and, if Russia invades Ukraine, we would look to contribute to any new NATO deployments to protect our allies in Europe."
The principle of collective defence is at the heart of NATO's North Atlantic Treaty, which sets out under Article 5 that "an armed attack against one or more [NATO ally] in Europe or North America shall be considered an attack against them all."
Watch: What happens if NATO triggers Article 5?
"We will not reopen that divide by agreeing to overturn the European Security Order because Russia has placed a gun to Ukraine's head, nor can we accept the doctrine implicit in Russian proposals that all states are sovereign, but some are more sovereign than others," the Prime Minister added.
"The draft treaty published by Russia in December would divide our continent once again between free nations and countries whose foreign and defence policies are explicitly constrained by the Kremlin in ways that Russia would never accept for herself."
He said when he spoke to President Putin in December he stressed NATO has no thought of encircling or threatening Russia.
Mr Johnson said: "I said that any attack on his neighbour would be followed by tougher sanctions against Russia, further steps to help Ukraine defend herself and by an increased NATO presence to protect our allies on NATO's eastern flank."
Conservative chairman of the Defence Committee Tobias Ellwood has said it is not too late to mobilise a "sizeable" NATO presence in Ukraine, utilising the alliance's power to "make Putin think twice".
Watch: Why is Ukraine not part of NATO?
Speaking in the House of Commons, the MP for Bournemouth East and former soldier said: "As the Prime Minister articulates, the West is now regrouping.
"But the penny is also dropping. The threat of sanctions will not deter a Russian aggression, and a total or even partial invasion will have severe economic and security consequences felt right across Europe and beyond."
He added: "So I ask the Government to liaise with the United States and now consider a more simpler and effective option to deter this invasion by belatedly answering Ukraine's call for help.
"It is not too late to mobilise a sizeable NATO presence in Ukraine, utilising the superior hard power the alliance possesses to make Putin think twice about invading another European democracy."
Prime Minister Boris Johnson replied: "I have to tell him that I don't believe that to be a likely prospect in the near term. Ukraine is not a member of NATO. But what we can do, and what we are doing is sending troops to support Ukraine."
At the Commons Foreign Affairs Committee, the Defence Secretary indicated that Russia already had operatives in place to prepare for an invasion.
"We are becoming aware of a significant number of individuals that are assessed to be associated with Russian military advance force operations that currently are located in Ukraine," Mr Wallace told MPs.