Sir Keir Starmer said MPs will have the opportunity to vote on the final number of troops deployed to Ukraine in the event of a peace deal.
British and French forces will help to train Ukrainian troops and protect stocks of weaponry, aimed at deterring future Russian aggression, following a joint agreement signed in Paris.
The Prime Minister told the Commons the number of personnel placed in “military hubs” in Ukraine will be determined in accordance with the UK’s military plans.
The agreement was revealed following a meeting of the coalition of the willing on Tuesday.
The US has signalled it will provide a security guarantee to the European-led peacekeeping efforts, something which Donald Trump has previously appeared reluctant to do.
US special envoy Steve Witkoff said the president “strongly stands behind” a security guarantee, though details of how this would work were not revealed.
Sir Keir told the Commons on Wednesday there is no difference between the UK and US over security guarantees for Ukraine.
Conservative leader Kemi Badenoch criticised Sir Keir for not making a statement to Parliament on the agreement, claiming this showed a “fundamental lack of respect” for MPs.
Opening the first PMQs of the year, Sir Keir said: “Yesterday, I stood side by side with our European and American allies and President Zelensky at the coalition of the willing meeting in Paris.
“We made real progress on security guarantees, which are vital for securing a just and lasting peace.
“Along with President Macron and President Zelensky, we agreed a declaration of intent for the deployment of forces in the event of a peace deal. We will set out the details in a statement at the earliest opportunity.
“I will keep the House updated as the situation develops and, were troops to be deployed under the declaration signed, I would put that matter to the House for a vote.”
He added that a statement would be made to the House “at the earliest opportunity”.
Mrs Badenoch suggested Sir Keir wanted to dodge questions from MPs, adding: “Why is today not the earliest opportunity?”
She then pressed the Prime Minister to “confirm how many troops would be sent to Ukraine and whether they would be in a combat role”.
He replied: “I will be clear with the House that there would only be deployment after a ceasefire, it would be to support Ukraine’s capabilities, it would be to conduct a deterrence operations and to construct and protect military hubs.
“The number will be determined in accordance with our military plans, which we are drawing up and looking to other members to support. So the number I will put before the House before we were to deploy.”
He reiterated if there was a “legal instrument” required, he would have a debate and vote in the Commons.
The Prime Minister later added: “On the question of security guarantees, there is nothing between the UK and US and we’ve been constantly discussing this over many many weeks and months.”
Elsewhere, European leaders and the Trump administration remain at odds over the US president’s desire to take over Greenland, the vast Arctic island which is an autonomous territory of Nato ally Denmark.
Using the US military is “always an option” for the president, the White House said of the proposed annexation on Tuesday, after it revealed Mr Trump has been discussing a “range of options” to acquire Greenland.
In the Commons, Mrs Badenoch called for an urgent meeting of Nato leaders.
Sir Keir did not commit to this, but said: “Nato is the single most important and effective military alliance the world has ever seen.”
He had joined European leaders in rallying alongside Denmark earlier on Tuesday, insisting they will “not stop defending” Greenland’s territorial integrity.
Danish PM Mette Frederiksen warned America seizing her country’s territory could spell the end of Nato.
Deputy Prime Minister David Lammy will meanwhile continue discussions over Ukraine with US vice president JD Vance in Washington DC.
Mr Lammy is travelling to Washington to take part in celebrations to mark the 250th anniversary of the United States.
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