A Russian invasion of Ukraine could see the worst bloodshed since WWII
The Prime Minister said he had agreed in a virtual meeting on Monday night (with US President Joe Biden, European leaders, and NATO general secretary) to respond "in unison to any Russian attack on Ukraine".
The world leaders agreed to impose "co-ordinated and severe economic sanctions, heavier than any we have done before against Russia," Mr Johnson told the House of Commons.
The PM also confirmed Britain is prepared to deploy troops to protect NATO allies in Europe should Russia invade.
The PM warned Russian presence on the Ukrainian border was "far bigger than anything Russia has deployed against her before," with more than 100,000 troops.
"Tragedy" would ensue should Russia move to invade, Mr Johnson warned, adding: "No one would gain from such a catastrophe."
What did the PM outline in his statement - and how does it relate to domestic politics? ITV News Correspondent Rachel Younger explains
Will the British Army be deployed to fight Russia in Ukraine?
The UK has been involved with NATO, the US and allies in Europe for the past months to try and find diplomatic solutions to the crisis in Ukraine.
"We stand ready as we always have to address any legitimate Russian concerns through honest diplomacy," Mr Johnson told MPs on Tuesday.
The PM said when he spoke to Russia's President Putin in December he stressed NATO has no thought of encircling or threatening Russia.
"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," Mr Johnson said.
On whether British troops would be deployed to defend Ukraine in the event of invasion, Mr Johnson told MPs: "The British Army leads the Nato battle group in Estonia and if Russia invades Ukraine, we would look to contribute to any new Nato deployments to protect our allies in Europe."
Earlier, Foreign Secretary Liz Truss failed to rule out UK combat troops being sent to help defend Ukraine – but told MPs such a scenario is “unlikely”.
Responding to the PM's statement, Labour leader Sir Keir Starmer said: "The Labour Party supports the steps that the government has taken to bolster Ukraine's ability to defend herself."
The opposition leader called on the PM, however, to assure MPs "that during talks, the UK and our partners will be resolute in our defence of Ukraine’s sovereignty, and the security of our NATO allies?"
Sir Keir added: "For too long the implicit message to Moscow has been that President Putin can do what he likes and the West will do little to respond".