The Defence Secretary said it was important for his mental health that he exited politics “on a high”.

Ben Wallace has announced he will leave the Cabinet at the next reshuffle and will step down as an MP at the next election.

Setting out his reasons for going, Mr Wallace said he wanted to give Prime Minister Rishi Sunak notice ahead of next year’s likely poll contest.

He told the Future of Britain conference, organised by the Tony Blair Institute for Global Change, that Mr Sunak was “a bit surprised” to hear he would be quitting politics, but said the Conservative Party leader understood his reasons.

I think it is probably really important for the mental health of myself and my family that you leave on a high

Mr Wallace, speaking moments after setting out the details of the armed forces Command Paper in the House of Commons, said that his time as security minister and then Defence Secretary had been eventful and that the last seven years had been “24/7”.

With an election expected in 2024, Mr Wallace’s Wyre and Preston North, which he has represented since 2005, has effectively been abolished in a constituency boundary review. He has opted not to find a new seat.

The senior Conservative politician suggested he had learned from Sir Tony’s exit from Downing Street, with the former prime minister standing down in 2007 after a decade in power, when announcing his own decision to resign.

“There is a man over there that inspires me to go on your own terms,” said Mr Wallace, gesturing to where the former Labour leader was sitting in the audience.

Pressed on why he was giving up his Defence Secretary post while the Ukraine conflict was still going on, the former Scots Guard said: “Ultimately I wasn’t going to stand at the next election.

“I think it is only fair to the sitting Prime Minister that he goes into the next election with a defence secretary who is not going to say six weeks before: ‘I’m off’.

“And I wanted to go on my terms. I think it is probably really important for the mental health of myself and my family that you leave on a high.”

Mr Wallace said it was “probably politics’ dirty secret” that the job of an MP had become less desirable, with “much” fewer applications to stand as a candidate compared with past elections.

“I remember safe Tory seats would have 600 applicants,” he said.

“A friend of mine from the North West Labour Party said a few years ago that in a really solid Labour seat, they had less than 100 applicants, only 20 or 30.

“Go back 30 years, that never would have happened.”

He said the demands of the media and the public were some of the factors at play behind the elected role losing attraction.

Meanwhile, Mr Wallace said he thought Kyiv could win in its struggle against Russia’s invading troops.

He told the conference: “I think it is winnable. I think Russia is much more fragile than the Russians want to admit.

“If you look at the generals being fired, if you look at (Wagner leader Yevgeny) Prigozhin… the splinter in the hierarchy of the Russian army is very real.

“The casualty rates are horrendous. It would not be wrong to say at least 230,000 to 250,000 dead or injured Russians.”

The Cabinet minister said the job of Nato general secretary would have been a position that would have been a “nice to have” but denied that he had set his sights on the job.

He did not respond to a suggestion that the US had blocked his candidacy but said he thought Washington wanted a former head of state for the role.

Mr Wallace said he could not thank Mr Sunak enough for campaigning among allies for him to be the next Nato chief but suggested the 31 member states were looking for a different type of candidate, perhaps a female leader.