The UK “will not flinch” over Ukraine and sanctions would be introduced if Russia invades, the PM has said.
Boris Johnson told Moscow that support to Europe and Nato “will remain unconditional and immovable”.
Writing in the Times, he said the government would ask for new powers to target “individuals and entities” linked to the Russian state.
Talks aimed at de-escalating the crisis have been held in Kyiv, where Emmanuel Macron is meeting Ukraine’s president.
The French president met Vladimir Putin on Tuesday, and said his Russian counterpart assured him Russian forces would not be ramping up the crisis near Ukraine’s borders.
Moscow has denied any plans to invade Ukraine but the West has become increasingly concerned by the possibility of a conflict.
Mr Johnson also wrote in the Times that the UK is considering deploying RAF fighters and Royal Navy warships to the region but said he hoped diplomacy could defuse the crisis.
Western countries have rejected a number of Moscow’s demands, including ruling out Ukraine becoming a member of the Nato defence alliance, and that it reduce its military presence in eastern Europe.
The prime minister said the immediate priority was “to stand with our allies and combine deterrence with dialogue to de-escalate this crisis and prevent the catastrophe of another Russian invasion of Ukraine”.
It is an urgent task, he added, as Moscow mounts the biggest military build-up in Europe since the Cold War.
Russia has massed an estimated 100,000 troops, tanks, artillery and missiles near its borders with Ukraine.
Mr Johnson said he wanted Europe and the US to have its economic sanctions ready “the moment the first toecap of the first Russian soldier crosses further into Ukraine’s sovereign territory”.
“For our part, British sanctions and other measures will be ready for any renewed Russian attack,” he wrote. “The government will ask Parliament for new powers to sanction a wider range of Russian individuals and entities, including any company linked to the Russian state or operating in a sector of strategic importance to the Kremlin.”
Foreign Secretary Liz Truss had said the new powers would not impose sanctions automatically, but “in the event of any further Russian incursion into Ukraine”. The measures would allow the UK to freeze assets and ban travel, the Foreign Office said.
In addition, the UK – along with Nato – is “willing to send more forces to guarantee the security of our allies on the eastern flank”, said Mr Johnson.
He said Estonia had asked for further support and “we stand ready to provide it”, with the deployment of RAF Typhoon fighters and warships being considered, and aircraft carrier the HMS Prince of Wales acting as the command ship of Nato’s Maritime High Readiness Force.
The prime minister said he did not understand how threatening to invade Ukraine would advance President Putin’s goals of keeping Nato forces from Russia.
“If he launches another invasion, he will force the West to bring about much of what he seeks to prevent,” he wrote.
And he said Ukraine wanted to be able to choose its destiny and seek its own alliances, adding: “Britain has always stood for these principles and we will not flinch now.”
Mr Johnson met Ukraine’s president Volodymyr Zelensky in Kyiv last week, saying the pair would work together to find a diplomatic solution to the row with Russia and “avoid further bloodshed”.
Ahead of his visit, the UK government announced it was giving Ukraine £88m to promote stable governance and energy independence from Russia.