Britain s Prime Minister Boris Johnson. AFP
Russian President Vladimir Putin's recognition of the independence of pro-Russian separatists in Ukraine is "a repudiation of the Minsk process and the Minsk agreements," Johnson said, referring to a faltering Ukraine peace deal.
"The UK will continue to do everything we can to stand by the people of Ukraine with a very robust package of sanctions," Johnson told journalists at Downing Street.
"They will be triggered with the first toecap of a Russian incursion or Russian invasion. But plainly what has happened is extremely bad news.
"We will be urgently talking to our friends and allies around the world, all of whom are jointly signed up with us in this package of sanctions."
"It is becoming clear that we're going to need to start applying as much pressure as we possibly can because it is hard to see how this situation improves," Johnson said, adding that he would call Ukrainian President Volodymyr Zelensky.
Putin's move could ignite a potentially devastating conflict with Kyiv's Western-backed government.
The recognition will effectively end the already shaky peace plan in the separatist conflict in Ukraine's east. It paves the way for Russia to move in troops to protect hundreds of thousands of residents in the regions who have been granted Russian passports.
It overshadowed last-ditch diplomatic attempts to ease weeks of tensions over fears Russia has been planning an all-out invasion of its pro-Western neighbor.