The Foreign Secretary will declare that Britain has the "strategic endurance" to see Ukraine through to victory over Russian invaders, in his Conservative Party conference speech.
James Cleverly will say that Ukraine has Britain's unwavering support in its efforts to push back Vladimir Putin's forces, saying that "we are players on the pitch" and not just "commentators".
It comes after Britain ramped up sanctions against Moscow, with new measures targeting vulnerable sectors of the economy, in response to the Russian president's "illegal" annexation of swathes of Ukrainian territory.
Mr Cleverly will repeat Prime Minister Liz Truss's vow that the UK will never accept Mr Putin's claim to the regions of Donetsk, Luhansk, Kherson, Zaporizhzhia or Crimea.
The Foreign Secretary is expected to say: "We aren't commentators. We are players on the pitch. Making a difference. Promoting our values. Competing on the world stage for what we believe is right.
"We believe in freedom. We believe in the rule of law. We believe that an aggressor cannot invade its neighbour with impunity.
Watch: Ukraine will have to take the fight to the Russians if they are to win, says urban warfare expert.
"This is why we stand shoulder to shoulder with those brave Ukrainians defending their homeland.
"And Britain has the strategic endurance to see them through to victory."
He will say that Ukrainians have defended their homeland "ferociously" and that their bravery has been "amplified" by arms and training supplied by the UK.
"We will support them until this war is won. We will support them until their sovereignty is restored.
"We will never recognise the annexation of Luhansk, Donetsk, Kherson, Zaporizhzhia or Crimea.
Watch: Satellite images show large numbers of Russians fleeing to Georgia.
"They are Ukraine. And when Ukraine has won this war then we will support them as they rebuild their homes, their economy, and their society."
The latest move by Russia, currently under pressure after a series of Ukrainian successes on the battlefield, has prompted condemnation globally and has been rejected by Ukrainian President Volodymyr Zelensky.
Mr Putin recently threatened to use "all the means at our disposal" to protect his country, seen as a sign that he could use tactical nuclear weapons in response to attacks on parts of Ukraine he has annexed.
In a further sign of escalation, the Russian president also announced a partial military mobilisation.