Wallace has been in the UK parliament for 18 years, and is the longest-serving Conservative defence secretary since Winston Churchill.

Reports suggested that Energy Security and Net Zero Secretary Grant Shapps, seen arriving at the 10 Downing Street office, was likely to be announced as his replacement.

In a letter to Wallace, 53, Prime Minister Rishi Sunak praised the "dedication and skill" he brought to the role that saw him take a leading role in Western allies' support for Ukraine against Russia.

"You have served our country with distinction," Sunak wrote, adding that he had seen "before others did what Vladimir Putin's true intentions in Ukraine were".

"Your determination to get Kyiv weaponry before the Russians attacked had a material effect on the ability of the Ukrainians to thwart the invasion."

Wallace, a former army officer and a close ally of former prime minister Boris Johnson, had been the UK's pick to succeed Jens Stoltenberg as NATO secretary general.

But he failed to get crucial US backing to replace him, and Stoltenberg has now extended his term at the head of the alliance.

He said after announcing his intention to resign in July that the decision was not because he thought the ruling Conservatives -- currently trailing the main opposition Labour party in the polls -- would lose the next election, but because his constituency in northwest England was being scrapped under boundary changes.

- Johnson ally -

He was the only minister in a senior post to remain in the turbulent transition from his political ally Johnson to the short-lived Liz Truss and then Sunak.

He was security minister under Theresa May before becoming defence secretary in 2019.

He had enjoyed strong support among the Tories' grassroots membership and was regularly tipped to be party leader but never actively ran for the top job.

Wallace said in an interview in July that he counted among his achievements boosting the defence budget by pound24 billion ($31 billion) and said higher defence spending would be crucial in the years ahead.

He predicted the world will be "much more unsafe, more insecure" by the end of the decade.

"I think we will find ourselves in a conflict. Whether it is a cold or a warm conflict, I think we'll be in a difficult position," he added.

The UK could be dragged into conflict in Africa against Islamist groups, he suggested, and voiced concern about the effect of Chinese expansionism in the South China Sea on regional politics, and nuclear proliferation.

On Ukraine, he said Russian President Vladimir Putin could "lash out" if he loses and would look for fresh targets, such as against undersea cables carrying Western communications and energy supplies.