Britain and other Western powers should provide warplanes to Ukraine, Foreign Secretary Liz Truss will say, as she calls for increased defence spending.
In a speech in London on Wednesday, Ms Truss will argue that the West "must be prepared for the long haul and double down on our support" for the country.
Since Russia's invasion, Ukraine has repeatedly asked allies to supply heavy armaments such as aeroplanes and tanks.
Nato has provided mostly only lighter weaponry amid fears of escalation.
Ms Truss will tell an audience at Mansion House that "the fate of Ukraine remains in the balance" and that the West "cannot be complacent".
"If Putin succeeds there will be untold further misery across Europe and terrible consequences across the globe. We would never feel safe again," she will say.
The speech comes just days after Polish Prime Minister Mateusz Morawiecki confirmed his country had sent tanks to Ukraine. Germany has also pledged to send about 50 anti-aircraft tanks.
Liz Truss's call for Ukraine to be given warplanes follows a growing trend. Western powers are ratcheting up military support for the country. Drones, long range artillery, better anti-aircraft defence systems.
So why not fighter jets? Well, some countries have said they were willing to give Ukraine some aging MIG29s. But they were reluctant to do so unilaterally for fear of provoking Russia.
Deals to share responsibility across Nato failed. As of last week, the Pentagon said that while Ukraine might have received spare parts, it had not received an entire aircraft from any country. Ukraine would dearly love some ground attack warplanes to target Russian armour and artillery.
But that appears unlikely until such time that the scale of Western support for Ukraine is such that aircraft become a natural next step and the need to help Kyiv outweighs the risk of Moscow escalating the conflict.
Ms Truss will also call for a boost in UK defence spending and argue that Nato's target of 2% of GDP - which the UK met last year - should be "a floor, not a ceiling".
UK Prime Minster Boris Johnson said on Tuesday he would be happy for Ukrainian forces to use British-supplied weapons to strike targets inside Russia to protect themselves.
Russian Foreign Minister Sergei Lavrov has said that, by arming Ukraine, Nato had already effectively "entered into a war with Russia through proxies".