Downing Street has insisted Liz Truss will not back down on a key commitment to boost defence spending after the armed forces minister publicly threatened to quit if new Chancellor Jeremy Hunt ditches the pledge.

Defence Secretary Ben Wallace would also be under pressure to follow James Heappey in resigning if the Prime Minister’s promise to spend 3% of national income on defence by 2030 was broken.

Mr Heappey said he and Mr Wallace, who is tipped as a potential successor to Ms Truss in Downing Street, both believe the pledge “must be delivered” and warned there is “no prosperity without security”.

No 10 went on to insist the Prime Minister is “committed” to the promise, but hinted at possible slower rises in defence spending ahead of the end of the decade.

Mr Hunt has refused to rule out rowing back on the promise as he searches for cuts to plug a multimillion-pound black hole in the nation’s finances.

A defence source said Mr Wallace believes the pledge, which the Royal United Services Institute think tank estimates would cost an extra £157 billion, is a “priority” and not a discretionary spend.

Mr Heappey warned Ms Truss cannot afford to make any more errors as he defended the Government after she apologised for “mistakes” in her tumultuous premiership.

He said he believes the Government still backs the spending target, but asked if he would quit in the face of a U-turn, he told LBC: “Yeah.

“But no-one has said that 3% is not going to happen by 2030.”

Mr Heappey insisted he would quit if that changed though, saying: “Yeah, we need to be spending 3% of our GDP on defence of our nation by 2030 because there is no prosperity without security.”

Levelling up minister Paul Scully told BBC Radio 4’s PM programme Mr Wallace and Mr Heappey would have done “pretty well” in their roles if they were around to resign by that point, should the pledge not be met.

The Telegraph reported that Mr Hunt, brought in to shore up Ms Truss’s ailing premiership, will ask that savings are made in the Ministry of Defence budget.

Mr Heappey, speaking to Sky News, said: “The commitment the Prime Minister made is 3% by 2030 and to be clear, like the Secretary of State (Mr Wallace), that’s something that I believe must be delivered given the need to keep our nation safe given increasingly uncertain times.

“If in the very immediate term there is a requirement to look at what we can do to help the Treasury out, that’s a discussion for the Chancellor to have with the Secretary of State.

“But I am confident that the Prime Minister and the Chancellor, and the Foreign Secretary, and frankly everyone else around Government, understands the importance of investing in our nation’s armed forces and our defences at a time when the UK interest at home and abroad is under such threat.”

Later in the day, No 10 insisted Ms Truss stands by her promise but conceded the “shape of that” increase could change.

The Prime Minister’s official spokesman said: “You’ve heard from the Chancellor on this.

“We are obviously committed to maintaining the UK’s position at the forefront of Nato, that’s why the PM committed to raise defence spending to 3% of GDP by 2030.

“The shape of that increase will be set out at future spending reviews in the normal way.”

Meanwhile, Mr Wallace insisted he wants to remain as Defence Secretary rather than run as a unity candidate to succeed Ms Truss, as suggested by some in Westminster.

In an interview with The Times, he pleaded with Tory colleagues to “stop playing political parlour games”.

“I want to be the Secretary of State for Defence until I finish. I love the job I do and we have more to do. I want the Prime Minister to be the Prime Minister and I want to do this job,” Mr Wallace added.

“I say to the colleagues who think our role is to feed the instability within the party, by proposing other people as leaders no matter who they are, (you) are doing a disservice.”

On Monday, Mr Hunt refused to rule out backing down on the defence spending commitment when questioned by MPs.

The Chancellor told the Commons he was “sympathetic” to the cause as he “campaigned for it when I was a backbencher very loudly and visibly”.

“But all of these things have to be sustainable,” he added.

At the Tory party conference earlier this month, Mr Wallace said he wanted to “hold the Prime Minister to account on her pledges to defence”.

“It was very important to me, when it came to the leadership election, that people recognised that defence spending is not a discretionary luxury at the bottom of people’s priorities, it’s for real,” he added.