Sir Keir Starmer spoke to Donald Trump on Wednesday and "reiterated the need for allies to stand together with Ukraine", Number 10 has said.
The prime minister said in a call with the US President-elect the support was necessary “in the face of Russian aggression and to ensure Ukraine is in the strongest possible position”.
A Downing Street spokesperson said in a statement that the call opened with the Prime Minister congratulating Mr Trump on the recent appointments to his team.
The statement went on: "Both agreed on their joint ambition to strengthen the close and historic relationship between the UK and the US. They looked forward to working together on shared priorities, including international security and delivering economic growth and prosperity.
"Turning to global conflicts, the Prime Minister reiterated the need for allies to stand together with Ukraine in the face of Russian aggression and to ensure Ukraine is in the strongest possible position.
"On the Middle East, the Prime Minister underscored the need to work together to ensure peace and security in the region.
"They agreed to keep in touch and looked forward to seeing one another at the earliest opportunity."
There have been concerns about whether the US will maintain its level of support for Ukraine when Mr Trump re-enters the White House in January.
The Presddent-elect suggested this week he could reverse permission for Ukraine to use American missiles inside of Russia.
Speaking at a press conference at Mar-a-Lago he said it was a “big mistake” to permit Ukraine to strike targets within Russia with US-supplied long-range missiles.
“Especially just weeks before I take over. Why would they do that without asking me what I thought? I think that was a big mistake,” he said.
He did not give a direct answer when asked at the press conference whether he believed Ukraine should cede territory to Russia as part of a negotiated settlement to the nearly three-year-old war.
On the Middle East, the incoming president said earlier in December that there would be "hell to pay" if Israeli hostages are not freed before he is sworn into office.
In a post on his Truth Social site, Mr Trump wrote: "Please let this TRUTH serve to represent that if the hostages are not released prior to January 20 2025, the date that I proudly assume Office as President of the United States, there will be ALL HELL TO PAY in the Middle East, and for those in charge who perpetrated these atrocities against Humanity."
Earlier this week, Sir Keir said that it would be a "big mistake" if Ukraine's allies did not continue to give it the "strongest" support.
He urged western allies to not "take our eye off the ball" as the third anniversary of the invasion approaches.
Speaking during a visit to Norway, Sir Keir warned it was time for Kyiv's allies to "double down" on the financial commitments, sanctions and the training they were providing to Ukraine.
"It's important that we put Ukraine in the strongest possible position if there are to be negotiations, and even if there aren't to be negotiations," he said.
"But it would be a big mistake, in my view, to take our eye off the ball and not ensure that Ukraine is in the strongest possible position, which is why we've been doubling down on this at many of the international meetings we've had with our allies to discuss this."
