Defence Secretary Liam Fox said he was "appalled" that his private warnings to the Prime Minister over the "grave consequences" of impending savage spending cuts were leaked to the media.
He expressed his anger as it was confirmed Ministry of Defence (MoD) Police were investigating the leaking to the Daily Telegraph of a private letter from Dr Fox to David Cameron.
In it he advised the Prime Minister that the risk of "seriously" damaging morale across the Armed Forces "should not be underestimated".
And he warned the Government could provoke a "brutal" international reaction if it failed to "recognise the dangers and continue to push for such draconian cuts at a time when we are at war".
Confirming that an investigation by MoD Police would take place, Dr Fox said: "It's appalling that a Secretary of State can't write to the Prime Minister in confidence and we will be looking into that this morning."
Labour accused the Government of being in "chaos" and voiced fears it was rushing its Strategic Defence and Security Review.
Shadow defence secretary Bob Ainsworth said: "This is an incredible letter which reflects many of our concerns about their approach to this review. Rushed decisions that are not based on our strategic needs pose a real risk for our future security."
General Sir Richard Dannatt, head of the Army between 2006 and 2009, said it was "most unfortunate" the "very private letter" had entered the public domain.
Sir Richard said a consensus was emerging that the number of fast jets will be "slashed hugely" and there is a question mark over the carrier programme. But he added: "If the UK wants to go on playing a significant role in the world, it has got to have some sort of power projection."
In the letter, Dr Fox set out specific spending implications and warned that losing amphibious landing ships would leave the country unable to mount missions such as that in Sierra Leone 10 years ago. The Navy could be forced to withdraw from areas such as the Indian Ocean, Caribbean or Gulf, while a cancelled Nimrod MR4 reconnaissance plane programme created "some risk" to civil contingent capability around Mumbai-style terror attacks and the 2012 Summer Olympics, he warned.