The Ukraine crisis has intensified after Russian President Vladimir Putin said he would recognise two breakaway regions.
The decision to recognise Donetsk and Luhansk in eastern Ukraine was an "ill omen" and a flagrant breach of international law, Boris Johnson said.
The Prime Minister's considering whether the actions could trigger the imposition of fresh sanctions on Russia.
"This is plainly in breach of international law, it's a flagrant violation of the sovereignty and integrity of Ukraine," he said.
"It is a repudiation of the Minsk process and the Minsk Agreements.
"I think it’s a very ill omen and a very dark sign."
Watch: UK 'must prepare' for the consequences of Russia invasion into Ukraine.
He added it was "yet another indication that things are moving in the wrong direction in Ukraine".
The move appears to have dashed hopes for a diplomatic breakthrough which had been raised with the possibility of talks between Mr Putin and US President Joe Biden.
Boris Johnson had previously said sanctions would be triggered if Russia invaded Ukraine.
"Plainly what has happened is extremely bad news and 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," he said.
Mr Johnson said it was "becoming clear that we are going to need to start applying as much pressure as we possibly can".
"It is hard to see how this situation improves," he acknowledged.