The PM also addresses Russian people and says he doesn't believe the attack is being done in their name.
Boris Johnson has said there will be a "massive package of economic sanctions" designed to "hobble" the Russian economy.
Addressing the Russian people, the PM said he could not believe the military action in Ukraine, ordered by President Putin, "was being done in your name".
President Putin launched what the Ukrainian government says is a "full-scale invasion" of the country. Ukraine has said Russian military vehicles have been entering the country from the north and the east. There have been explosions in the Ukraine's capital Kyiv, and reports of missile strikes in other cities.