Ben Wallace confirmed his resignation as British defence minister on Thursday in a letter to Prime Minister Rishi Sunak, offering the government his continued support while warning the prime minister not to see defence as a "discretionary spend". Ex-energy secretary Grant Shapps has now been named as the country's new defence minister.

Wallace has served as defense secretary under three prime ministers and played a key role in the UK's response to Russia's invasion of Ukraine. A former captain in the British army, Wallace, 53, was appointed as defence minister in 2019 by his friend and ally, former Prime Minister Boris Johnson, after holding junior ministerial roles in earlier governments.

Wallace had said last month that he would stand down as a lawmaker at the next Cabinet reshuffle. He had told The Sunday Times last month his departure was due to the strain his job had put on his family. He also said he would stand down as a lawmaker at the next general election.

"Last month marked my fourth year as Secretary of State for Defence. It also marks the ninth year as a Minister. I have had the privilege of serving you and your predecessors in the task of protecting this great country and keeping its citizens safe. As you know that responsibility carries with it a 24*7 duty to be available at almost no notice. In my time as both security Minister and at Defence, I have been able to contribute to the Government's response to a range of threats and incidents," he said in his resignation letter.

Wallace, who had been touted as a potential successor to NATO Secretary General Jens Stoltenberg, had taken a leading role in shaping Britain's response to Russia's invasion of Ukraine last year.

He is the longest continuously serving minister in government. He was security minister under former Prime Minister Theresa May, before being promoted to defense secretary by her successor Boris Johnson.

Wallace drew criticism last week when he suggested that Ukraine should show “gratitude” for the West's military support. He made the remark at the NATO summit in Lithuania after Ukrainian President Volodymyr Zelenskyy expressed frustration about when his country could join the military alliance.

British Prime Minister Rishi Sunak distanced himself from Wallace's comments, saying Zelenskyy had “expressed his gratitude for what we've done on a number of occasions.”

(With inputs from Reuters and the Associated Press)