Why is the defence spending plan so controversial?

Sir Keir Starmer was reportedly sad to lose the pair on the back of the fall out of the defence funding plan and has written to Mr Healey, stating it “will provide the resources our military needs to keep us safe”.

He has appointed Dan Jarvis as the new defence secretary as he looks to steady the ship after the departmental rebellion and vowed to continue to keep Britain safe.

"I am pleased to appoint Dan Jarvis as defence secretary as we strengthen our armed forces and meet the growing threats facing our country," the prime minister said.

"My first duty is to keep the British people safe, and I will always do what is necessary to protect our national security."

Read also: SAS veterans say government has broken contract with troops as Al Carns quits amid defence turmoil

What is the defence spending plan?

The defence spending plan, which is due to be unveiled soon, is slated to improve Britain's defence against emerging threats.

It was supposed to be published in the autumn, but is now set to be released ahead of a Nato summit early next month - although the hold-up has been criticised for allowing equipment costs to rise in the meantime.

Sir Keir said: "This Labour government is delivering the largest sustained increase in defence spending since the Cold War.

"In a dangerous and volatile world, we will give our armed forces the capabilities they need to defend Britain and keep our nation secure."

The prime minister has insisted he was increasing defence spending “in a sustainable way” and said “irresponsible borrowing” would put the country’s finances “at risk”.

Why has the plan been so controversial?

All three Labour frontbenchers cited the hated defence spending plan in their resignations, suggesting the plan will not prepare Britain for evolving threats.

John Healey resigned as defence secretary after claiming the money in the defence investment plan would not be sufficient to defend the UK.

He wrote to the PM: "You agreed to spend 3.5% of GDP in 2035 through the next Spending Review.

"As we have regularly discussed, I am certain that a headmark date for 3% of GDP on defence in 2030 is what Britain must set."

He added: "Your DIP financial settlement falls well short of what is required for defence and the country at this dangerous time.

"The extra support is backloaded when the pressure of operations and imperative to speed up readiness to fight is in the first two years and it rises to just 2.68% of GDP in 2030, when we will reach 2.6% next year with the investment we are already making."

Pamela Nash later resigned as parliamentary private secretary (PPS) to Mr Healey, while Al Carns then stepped down as armed forces minister.

Mr Carns said he watched on as the defence secretary remained locked in a standoff with Chancellor Rachel Reeves, who he accused of refusing to properly fund defence.

“What we really need is an honest discussion with the population," he told LBC's Nick Ferrari.

"As to where that money needs to come from and use really innovative ways and every means possible to get the right money for defence.”

Technology Secretary Peter Kyle told LBC the Prime Minister is paying to attention to defence.

"Just look at the facts though," he told Nick Ferrari.

"The cash has increased. Keir Starmer has increased the defence spending at a rate that nobody thought was deliverable.

"He has got to 2.5%, then to 2.6%, years ahead of the target. And we've done so on the back of a growing economy.

"The Tories cut the percentage to 2.3% at the back of a shrinking and a stagnating economy, so overall it meant even less money going into our armed forces. We are getting back to where we need to be."