The new Prime Minister risked a clash with the US president by raising the sensitive issue of Elon Musk's satellite system during a phone call on Friday.
Starlink provides high-speed internet crucial for battlefield communications and drone warfare.
But Kyiv says its access has been restricted as Trump has moved closer to Putin over the war, raising fresh questions about Washington's support for Ukraine.
In a statement following the call, a Downing Street spokeswoman said: "The Prime Minister spoke to the President of the United States of America Donald Trump today.
"The Prime Minister underlined the importance for Ukraine to have full access to Starlink so they can continue to defend themselves against Russia's full-scale invasion.
"The Prime Minister recognised the work of President Trump to bring Putin to the negotiation table and the leaders agreed to work together, alongside international partners to bring an end to the bloodshed."
Mr Musk has suggested the White House ordered the restrictions on Starlink, but the Wall Street Journal reported that the tech billionaire himself was behind the slowdown.