Rishi Sunak is facing pressure from President Volodymyr Zelensky and Boris Johnson to supply Ukraine with long-range missiles to help end the Russian invasion.
The Ukrainian president joined a call with the Prime Minister and other G7 leaders including Joe Biden to demand the weaponry to hasten his victory over Vladimir Putin.
Downing Street indicated the UK is not planning to send long-range missiles, which could give Kyiv the ability to launch strikes in Russia.
Mr Zelensky asked the virtual G7 meeting on Monday for “modern tanks”, as well as more guns and shells.
“We need more rocket artillery and more long-range missiles. The more effective we are with such weapons the shorter the Russian aggression will be,” he added.
He warned that failing to protect the Ukrainian energy sector from Russian missiles and Iranian drones will provoke a “migration catastrophe” not just for Ukraine but for Europe.
Earlier in the day, Mr Johnson spoke in the House of Commons to call for the UK to supply Kyiv with long-range missiles.
The former prime minister said he believes Ukraine should receive systems such as ATACMS, which have a range of up around 190 miles, to “bring the war to an end as soon as possible”.
Defence Secretary Ben Wallace responded: “I constantly review the weapon systems we could provide. I hear his call on ATACMS from the United States.
“We, too, have in our armoury potential weapon systems that are longer and, should the Russians continue to target civilian areas and try and break those Geneva Conventions, then I will be open-minded to seeing what we do next.”
The Prime Minister’s official spokesman said the weaponry currently being supplied to Ukraine is “for use in Ukraine to enable it to defend itself against the illegal invasion”.
“I’m not aware of any plans to alter that approach but obviously we keep these things under review,” he added.
During the G7 call, Mr Sunak urged allies not to decrease their high spending in military support of Ukraine’s resistance so Mr Putin feels “the cost of his action”.
“The Prime Minister said it was clear Putin was realising he could no longer win on the battlefield and was now resorting to cynical tactics, including barbaric attacks on critical national infrastructure,” a Downing Street spokeswoman said.
Back in September, the UK committed to next year spending at least the £2.3 billion in military support given in 2022.
Mr Sunak called on the G7 allies – also including Germany, France, Italy, Japan and the European Union – to “combine their efforts and match their support to Ukraine in 2023”.
“Remaining aligned politically, economically, and militarily was the only way to be sure Putin felt the cost of his actions, the Prime Minister reflected,” No 10 said.