The Prime Minister told the Council of Europe summit that “the UK may have left the EU, but we have not left Europe” and Russia must be held “accountable for the horrendous war crimes that have been committed”.
The comments come a day after Ukrainian President Volodymyr Zelensky met Mr Sunak at Chequers and Downing Street announced that the UK is set to provide war-torn Ukraine with air defence missiles and attack drones.
Mr Sunak told the summit in Reykjavik, Iceland: “Friends, the UK may have left the EU, but we have not left Europe.
“We remain a proud European nation.
“And we must work together to defend the values we all hold so dear.
“The Council of Europe, with its huge reach, has such a vital role to play.
“And we need to think about how this council should react to the realities of today.
“We showed great purpose in expelling Russia last year – acting decisively together within days of the invasion.
“Let’s bring that dynamism to the issues before us now. And let’s send a message from this hall, loud and clear, that we will stand by Ukraine for as long as it takes.
“We will hold Russia accountable for the horrendous war crimes that have been committed.”
He condemned Russia’s action as a “threat to democracy and the rule of law on our continent”, adding that with this act of aggression on European soil, and China growing in assertiveness, “the world is becoming more contested and more volatile”.
Earlier on Tuesday Mr Sunak agreed with Dutch leader Mark Rutte to “work to build an international coalition to provide Ukraine with combat air capabilities, supporting with everything from training to procuring F16 jets”, Downing Street said.
A spokesman added: “The Prime Minister reiterated his belief that Ukraine’s rightful place is in Nato and the leaders agreed on the importance of allies providing long-term security assistance to Ukraine to guarantee they can deter against future attacks.”