Shadow defence secretary Liam Fox said front-line services would not be cut
The Tories have asked civil servants to draw up plans to cut Ministry of Defence costs by 25% without reducing front-line troops.
Shadow defence secretary Liam Fox told the party's conference: "Some things will have to change and believe me, they will."
The BBC's Laura Kuenssberg said Mr Fox had asked for savings in bureaucracy - the MoD has 85,000 civil servants.
The Tories have pledged to cut overall Whitehall budgets by a third.
In his speech to party members in Manchester, Dr Fox accused Labour of creating "a black hole" in defence budgets, which was affecting the war in Afghanistan and threatening to create an "on-going defence crisis for years to come".
'Efficient as possible'
He also pointed to the need to redress the balance between civil servants and military personnel.
He said: "Frederick Duke of York was preparing for the Napoleonic threat between 1792 and 1804 he increased the size of the Army from 50,000 to nearly 500,000 - and he did it with 38 staff at Horse Guards.
It's not a crime to bring weight and experience into government with you
This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.