Troop numbers are kept under "constant review" should the situation in Ukraine continue to change, the Prime Minister has told the House of Commons.
Boris Johnson was responding to a question from Conservative MP and British Army veteran Johnny Mercer.
The former Veterans Minister asked if Mr Johnson could confirm that "no directive has been issued from No.10 or the Treasury on defence numbers, on numbers of personnel, and that will continue to be the case going forward should the situation (in Ukraine) continue to change?"
The Prime Minister said: "We keep the actual numbers under constant review.
"The most important thing, Mr Speaker... our troops are the best in the world but they've got also got to have the best equipment in the world and that's what we are paying for."
Mr Mercer said on Twitter that it was "an important commitment from the PM".
He tweeted: "An important commitment from the PM just now [in] the House that troop numbers will be kept under review as the situation changes.. giving flexibility to grow in the years ahead. Mass matters."
Last month, the Prime Minister committed Britain to increase defence spending to 2.5% of GDP by the end of the decade as NATO rises to the threat of Russian aggression.
Mr Johnson pledged the hike after a Cabinet row over defence spending, and insisted that the "cost of freedom" is "always worth paying".
Defence Secretary Ben Wallace reportedly wrote to the Prime Minister calling for a 20% increase in defence spending a year by 2028, to meet shortfalls in military capabilities.
An MOD spokesperson, in response to Mr Mercer's comments in the House, said: "Under Future Soldier, the Army will have a whole force of over 100,000 comprising a trained Regular Army of 73,000 and a total Army Reserve of over 30,000.
"We are investing an extra £24bn cash in defence over four years – the biggest investment in the UK's Armed Forces since the end of the Cold War – which will help provide the British Army with new armoured vehicles and attack helicopters, and upgraded tanks.
"The radical 'Future Soldier' transformation offers the best combination of people and equipment, given the available resources."
The spokesperson added: "It will make the Army more agile, lethal and expeditionary – ready to fight the wars of the future, not those of the past."