Britain’s leading role in providing weapons to Ukraine in its fight against Russia’s invasion will endure, whoever replaces Boris Johnson as prime minister, the country’s ambassador to Nato has said, amid concerns in Kyiv that a successor could fail to match his focus on the war.

Johnson’s enthusiastic support for Ukraine was one of the central features of his premiership this year, and Ukrainian president Volodymyr Zelenskyy said he had heard news of the prime minister’s decision to step down with “sadness”.

“We have played a very significant, leading role in Ukraine. We will continue doing so,” said David Quarrey, the UK’s permanent representative to Nato.

“Obviously, I cannot speak for decisions that ministers will make in the future, but it seems that there is very broad support at home for the role that we have played and indeed are continuing to play,” Quarrey told the Financial Times. “It hasn’t just started. And I don’t think it is just about to end.”

Eleven contenders are running to replace Johnson, after he announced his intention to step down as PM, in a leadership contest that is expected to take at least two months. Defence secretary Ben Wallace, who like Johnson has received credit for Britain’s support to Ukraine, has ruled himself out of the race.

Johnson said late last month that Britain would provide another £1bn worth of air defence systems, unmanned aerial vehicles and other weapons to Ukraine. That takes Britain’s total military support for Kyiv since Russian president Vladimir Putin launched his attack in February to £2.3bn, which London says is the largest amount after the US.

“The most important thing in all of this is that we give the Ukrainians the capabilities they need to defend their country,” said Quarrey, who took up his role in April after serving as Johnson’s adviser on international affairs since 2019. “And we will continue increasing our support, I’m sure, to help them continue doing so . . . in this fight.”

At Nato’s annual summit in Madrid last month, the alliance signed off on a new 10-year strategic doctrine and a new defence posture that will significantly beef up its defence in eastern Europe, after concluding that attempting to deter Russia is not sufficient, and a plan to repel any invasion is necessary.

That outcome came alongside a reiteration of support for Kyiv. Zelenskyy has repeatedly made clear that continued supplies of western weapons are critical in his army’s attempts to defend against Russia, and efforts to retake already captured territory.

“Some people had been concerned before Madrid, [there was] talk of Ukraine fatigue, would we stay the course,” said Quarrey. “The message I heard was completely the opposite. Allies were even more resolved.”

“I think that unity has deep roots. When I look at public support in the UK . . . that feels to me to be very resilient,” he added. “People know it comes with a cost. We are investing heavily to support Ukraine in the way that we are. But people know it is the right thing to do.”

Britain’s prominent role in spearheading European efforts to assist Ukraine and its troops has highlighted London’s commitment to the continent’s defence and security, European officials say, and silenced those who feared Brexit would result in the country playing a lesser role in wider European defence issues.

“Everybody can see from our leadership role on Ukraine, but not just on Ukraine, that indeed the opposite is the case. We are even more fully engaged in European security, and indeed global geopolitics and security . . . now than previously,” said Quarrey, a former British ambassador to Israel, who has also served diplomatic postings in New York, New Delhi and Harare. “I feel there is a very strong welcome here for that . . . including from Nato allies who are EU states.”

Nato’s increased importance and profile owing to the war in Ukraine, and Britain’s role within it, has also arguably earned the UK more credit in Brussels since the Brexit vote in 2016, and helped to cool talk inside the EU for more “strategic autonomy” and in-bloc military capabilities.

“I’m sure Nato has never been uncool,” said Quarrey. “But its relevance and importance is even more strong now than it has been for some time.”