Britain has ordered two new aircraft carriers
The first "root and branch" review of Britain's defence policy for 10 years has been announced by the government.
Defence Secretary Bob Ainsworth said he wanted to "ensure the armed forces are fit for the challenges of tomorrow".
He promised a Green Paper in early 2010 setting out initial thoughts followed by a more substantive Strategic Defence Review after the next general election.
Government sources said the initial review would not consider making cuts to the £36bn defence budget.
Instead, the review will examine defence policy as a whole - its purpose, the way it works, what the services expects, its technological priorities.
This means that the government appears to be putting off any substantive spending decisions about defence until after the election.
It will be the first full-scale review of the armed forces since the strategic defence review of 1998.
In a written statement, Mr Ainsworth said: "The government's current priority for the armed forces is to ensure they have the equipment and support they need for operations in Afghanistan.
"We have approved over £2.2bn from the reserve for urgent operational requirements in Afghanistan.
"Overall spending from the reserve, above costs met from the MoD budget, was over £2.6bn in the last financial year.
"But, in parallel, we must ensure the armed forces are fit for the challenges of tomorrow."
He said the first stages of the review would cover the "strategic context" for defence, including the lessons learned from recent operations and the "changing character of conflict".
It will also look at how the Ministry of Defence works with other government departments and the contribution the armed forces can make to international diplomacy and the projection of "soft power" to prevent conflicts.
The review will also examine changes in technology and internal processes, including acquisitions.
Chief of the Defence Staff, Air Chief Marshal Sir Jock Stirrup, said: "The 1998 strategic defence review has served us well.
"But much has changed since it was published, and it is right that we address with urgency the challenges facing defence in the future."
An influential think tank report published last month suggested the UK should consider slashing defence spending by up to £24bn and revisit plans to renew its Trident nuclear weapons system.
The Institute for Public Policy Research (IPPR) report said Britain cannot afford much of the defence equipment it plans to buy and urged the government to consider possible alternatives to Trident or extending the life of the system.
Its authors include former defence secretary Lord Robertson and the ex-Lib Dem leader, Lord Ashdown.
The government and the Conservatives back renewing Trident when it expires in 2024, at an estimated cost of £20bn, but many Labour MPs oppose it.
Lib Dem leader Nick Clegg has also called for it to be scrapped, saying it is too expensive and no longer meets the UK's defence needs.
The defence review also comes after news of a £1bn cost overrun on two new aircraft carriers.
The original budget for the two carriers for the Royal Navy was £3.9bn but the BBC has seen a memorandum revealing the programme will come under "severe pressure" because of the cost escalation.