Sunak said Britain was giving GBP 500 million ($617 million) in additional military funding for Kyiv as he held talks with Polish premier Donald Tusk and NATO chief Jens Stoltenberg.
Sunak said London would gradually boost defence spending to 2.5 percent of GDP by 2030.
"In a world that is the most dangerous it has been since the end of the Cold War, we cannot be complacent," Sunak told reporters in Warsaw.
The increase in spending from 2.3 percent will see the UK become one of the top spenders on defence in the 32-member NATO defence alliance after the United States, the British government said.
NATO countries are facing pressure to raise defence expenditure in the face of global threats, particularly from Russia and the possibility of an escalation in the Middle East.
Announcing the move, Sunak spoke of "an axis of authoritarian states", singling out Russia, Iran, North Korea and China.
"I believe we must do more to defend our country, our interests, and our values," Sunak said, adding the increase in spending would be "the biggest strengthening of national defence for a generation".
It means the UK is expected to spend GBP 87 billion on defence in 2030-31, an increase of GBP 23 billion over current levels.
- Missiles, armoured vehicles -
On Tuesday, Sunak also announced GBP 500 million additional funding for Ukraine's war effort against Russia.
Ukraine has for months faced a growing shortage of ammunition, with Europe struggling to provide weaponry as Ukrainian forces get pushed back along the front line.
US lawmakers eased the pressure last weekend by unblocking a $61-billion military aid package for Kyiv following six months of political wrangling.
EU defence and foreign ministers insist that Europe must speed up its deliveries of arms to Ukraine.
Germany has answered Kyiv's call in recent days by saying it would send an extra Patriot air defence system to Ukraine.
Stoltenberg said last week that he expected more NATO countries to make announcements on fresh air defences for Kyiv "soon".
During his trip, Sunak announced the UK's "largest-ever single package of equipment" for Ukraine's war effort, including more than 1,600 strike and air defence missiles and additional Storm Shadow long-range precision-guided missiles.
He also pledged more than 400 vehicles, including armoured ones, and nearly four million rounds of small arms ammunition.
- Sky Shield -
Ukraine has asked especially for seven more advanced Patriot systems capable of shooting down Russia's hypersonic missiles -- but is keen to get any help it can.
Six nations in the EU -- Germany, Greece, the Netherlands, Poland, Romania and Spain -- have the US-designed systems.
Sunak will now travel to Germany where he will meet Chancellor Olaf Scholz in Berlin on Wednesday.
The German government has led a push to bolster NATO's air defences in Europe amid Russia's relentless missile strikes on Ukraine.
Berlin has urged allies to buy deterrence systems together and the UK is among some 20 countries to have signed up to Germany's common air defence project, the European Sky Shield Initiative.
EU chief Ursula von der Leyen recently warned EU leaders to boost defence spending.
Brussels is set to come up with more proposals for financing the defence push by a summit of EU leaders in June.
It has put forward a 1.5-billion-euro ($1.6-billion) strategy to step up defence production, but officials say this is nowhere near sufficient.
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