Starmer said he had not taken the decision to consider putting British servicemen and women "in harm's way" lightly, but securing a lasting peace in Ukraine was essential to deter Russian President Vladimir Putin from further aggression.
US Secretary of State Marco Rubio on Sunday said Ukraine and Europe would be part of any "real negotiations" to end Moscow's war, signalling that US talks with Russia this week were a chance to see how serious Putin is about peace.
The end of Russia's war with Ukraine "when it comes, cannot merely become a temporary pause before Putin attacks again," Starmer wrote in the Daily Telegraph newspaper.
Starmer's comments were the first time he has explicitly said he is considering deploying British peacekeepers to Ukraine. He has previously said that Britain was willing to help play a part in any peace deal that is negotiated.
In the article, Starmer said he was prepared to contribute to security guarantees to Ukraine by "putting our own troops on the ground if necessary".
"I do not say that lightly," he wrote. "I feel very deeply the responsibility that comes with potentially putting British servicemen and women in harm's way."
Starmer is expected to join German Chancellor Olaf Scholz, Polish Prime Minister Donald Tusk, NATO Secretary General Mark Rutte, Italian Prime Minister Giorgia Meloni and other European leaders in Paris on Monday after French President Emmanuel Macron convened talks on Ukraine.
US President Donald Trump stunned European allies in NATO and Ukraine last week when he announced he had held a call with Putin without consulting them and would start a peace process. Trump's Ukraine envoy, Keith Kellogg, then suggested Ukraine and other European leaders would have no place at peace negotiations.
US and Russian officials are expected to meet in Saudi Arabia in the coming days to start talks aimed at ending Russia's nearly three-year war in Ukraine.
Starmer is expected to travel to Washington soon and he suggested on Sunday that Britain could play a "unique role" in the negotiations to end the war, acting as a bridge between Europe and the US during the peace process in Ukraine.
"Europe and America must continue to work closely together – and I believe the UK can play a unique role in helping to make this happen," he said.
"We are facing a once in a generation moment for the collective security of our continent. This is not only a question about the future of Ukraine. It is existential for Europe as a whole."
