BBC reported that the British embassy in the Ukrainian capital Kyiv will reopen next week, Boris Johnson has announced.
Ambassador Melinda Simmons said she was “heading back” after leaving when Russia invaded.
The prime minister also said there was a “realistic possibility” the Russian bombardment would continue until the end of next year.
Mr Johnson added the UK was looking at sending tanks to support Poland as it supplies Ukraine with heavy weaponry.
The prime minister made the embassy announcement at a news conference in Delhi, where he has been holding talks with Indian leader Narendra Modi.
It comes after Russia withdrew forces from around Kyiv when it failed to seize the capital, and launched an assault on the eastern Donbas region.
Mr Johnson visited Kyiv earlier this month to meet President Volodymyr Zelensky, following a pledge to give £100m worth of weapons to Ukraine.
Taking questions from reporters, the prime minister said intelligence assessments that the Russian assault could continue to the end of next year, and end with a Russian victory, were plausible.
He said Russian President Vladimir Putin has “a huge army” but “he has a very difficult political position because he’s made a catastrophic blunder”.
Mr Johnson said: “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.
“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 a long period, he will not be able to conquer the spirit of the Ukrainian people.”
The prime minister said he was looking at what the UK could do to “backfill” weapons in countries, such as Poland, “who may want to send heavier weaponry to help defend the Ukrainians”.
“We’re looking at sending tanks to Poland to help them, as they send some of their T-72s [tanks] to Ukraine and other steps like that,” Mr Johnson said