The Prime Minister said: "The sad thing is that is a realistic possibility.
"Putin has a huge army, he has a very difficult political position because he has made a catastrophic blunder.
"The only option he now has, really, is to continue to try to use his appalling, grinding approach driven by artillery, trying to grind the Ukrainians down.
"He is very close to securing a land bridge in Mariupol now. The situation is I'm afraid, unpredictable at this stage. We've got to be realistic about that.
"But what we've also seen is the incredible heroism of the Ukrainians and their willingness to fight.
"I think no matter what military superiority Vladimir Putin may be able to bring to bear in the next few months - and I agree it could be it could be a long period - he will not be able to conquer the spirit of the Ukrainian people. And that is just an observable fact."
"On the contrary, what he's doing every day is strengthening and reinforcing that will to resist in the people of Ukraine. What supporters of the Ukrainian people need to think about is what further steps we can do to support them militarily."
Mr Johnson said the UK was looking to send tanks to Poland to help "backfill" their military resources to assist with them sending heavier weaponry to help defend Ukraine.
The Prime Minister has also announced that Britain is to reopen its embassy in Kyiv again from next week.
The announcement follows his surprise visit to the city to meet President Volodymyr Zelensky earlier this month.
It comes as diplomats from other European countries have been returning to the capital following an easing of the security situation with the pull back of Russian forces which had been building for an assault.
Britain initially moved the embassy to the western city of Lviv in February shortly before the Russian invasion began.
Foreign Secretary Liz Truss then announced in early March that ambassador Melinda Simmons had left the country altogether due to the “serious security situation”.