John Healey has resigned as defence secretary in a letter to Prime Minister Keir Starmer.
In a letter to the prime minister, he says the defence investment plan "falls well short of what is required for defence and the country at this dangerous time".
Healey says he was given the financial settlement for the first time in full "on Monday afternoon this week".
Here are some of the key lines - we'll have more soon:
"This new era for defence required further investment through the Defence Investment Plan. The excellent and extensive cross-government work that completed in January-overseen by you, me and the Chancellor - confirmed the scale of the challenge and the rising demands on defence.
"Since then, you have been unable, and the Treasury has been unwilling, to commit the resources that the nation needs to defend the country at this time of rising threats.
"As I've outlined to you, there are credible ways of meeting the mid-term funding challenges, working multi-nationally and as other European nations are doing, to allow us to protect our ability to deliver the missions of our Labour Government.
"You know what defence needs. You made the argument for this powerfully in your speech at the Munich Security Conference back in February. Without a DIP (Defence Investment Plan) that meets the moment in this way, I am being forced to make decisions that would reduce the readiness of our Forces and increase the risk to personnel on operations, and could make the country less safe."
He outlines areas he is proud of, including leading on support for Ukraine during its war with Russia, and establishing Britain as a leading voice in Nato.
However, he complains about funding for the long-awaited Defence Investment Plan. He says the DIP financial settlement - "which I was first given in full on Monday afternoon this week" - falls "well short of what is required for defence and the country at this dangerous time".
John Healey says the financial settlement for the Defence Investment Plan (DIP) offered by the prime minister "falls well short of what is required for defence and the country and this dangerous time".
The DIP, which has not yet been released, will lay out government defence spending.
It will explain how new equipment and defence infrastructure will be funded over the coming decade, following a review of Britain's capabilities in June last year.
But publication has been delayed after the MoD reportedly asked for an extra £28bn over the next four years, following an internal assessment last year.
On Wednesday, Starmer told the Commons it was due to be released ahead of a Nato summit early next month.
Speaking on Monday, Healey told MPs that Starmer "is determined that we publish the defence investment plan before the Nato summit".
