Failure in Afghanistan is not an option, ministers say
Bob Ainsworth has hinted at "major shifts" in defence spending as the government prioritises resources for the war in Afghanistan.
The defence secretary urged a "wide-ranging" debate about future priorities but did not say what might be squeezed.
On Trident, Mr Ainsworth said the UK would "stick" with its seaborne nuclear deterrent but would look at its costs.
Several senior Labour figures, including Charles Clarke, have called for Trident not to be renewed.
The Liberal Democrats argue that the UK cannot afford to replace Trident with a like-for-like successor when it expires in 2024.
Ministers say they will begin a strategic defence review early in the next Parliament, the first since 1998, and will launch a green paper on future defence priorities early next year.
Opposition parties say the scale of the UK's military commitment in Afghanistan - where 9,000 UK troops are stationed - and the losses there mean this process should be accelerated.
In a speech in London, Mr Ainsworth said the Afghan deployment would remain the UK's "principal commitment for as long as it takes" and could not rule out knock-on effects for other operations as resources were focused there.
"We cannot exclude major shifts in the way that we use our defence spending to refocus our priorities," he said.
"There will be tough choices ahead."
Despite seeing real-term increases in spending in past years, Mr Ainsworth acknowledged there were now "significant pressures" on the Ministry of Defence's £36bn budget.
"We will need to be better at spending the money we have and more rigorous in prioritising what we spend it on."
The MoD has been criticised for going over-budget on major procurement projects and for not being able to account for spending on certain systems and equipment.
Future of Trident
Later, Mr Ainsworth said the future of Trident was not under review as MPs had backed its renewal in 2007, however he suggested during a Sky News interview that the cost of the project - which experts say will top £20bn - would be looked at.
"Of course, if we can provide that at sea deterrent with three submarines rather than four submarines, and therefore cut the cost, we will look to do that."
The Conservatives back Trident but say it will form part of an overall review of the defence budget which they plan immediately if they win the next election.
The Lib Dems have called for a smaller, cheaper deterrent to reflect the changing nature of warfare and threats to the UK.
A report by the think tank Reform, published on Tuesday, argued that not renewing Trident could save £70bn over 25 years.
A further £40bn could be trimmed off the defence budget by scrapping some hardware and reducing waste, it added.