National Security Council to meet over defence budget
The National Security Council will meet on Thursday to discuss key decisions on the future of the UK's defence budget.
Uncertainty remains over whether plans for both the UK's two proposed new aircraft carriers will go ahead.
There are fears the Royal Navy could cut many of its other surface ships if the orders are allowed to continue.
Ahead of the forthcoming Strategic Defence Review, David Cameron told the Conservative Party conference he would "take no risks with British security".
One of the Navy's new carriers is currently being built and defence industry sources say scrapping the second would not save money.
To build both will cost some £5.4bn, but thanks to cancellation fees it would cost more - £5.7bn - to build one but scrap the other.
That means building both - even if the second is then kept in port - could prove the most likely option.
However, the planned order for the joint strike fighters to go on board the carriers may have to be scaled down.
Former Prime Minister Gordon Brown has warned the government against cancelling orders for the carriers, HMS Queen Elizabeth and HMS Prince of Wales.
With savings of up to 10% being sought from the Ministry of Defence, Mr Cameron said there would be some big changes needed to the defence budget to modernise the armed forces which he said were "geared up to fight old wars".
The country had struggled to provide helicopters for troops in Afghanistan while maintaining armoured brigades designed for the Cold War, he said.
Mr Cameron restated his commitment to renew the UK's Trident-based nuclear deterrent and the withdrawal of British combat troops from Afghanistan by the end of 2015.
He told the conference in Birmingham: "Since becoming prime minister nothing has shocked me more than the catastrophic state of the defence budget.
"So our defence review will match our commitments with the resources we've got. This will mean some big changes.
"But I promise you this - I will take no risks with British security.
"That's why, when more and more countries have or want nuclear weapons, we will always keep our ultimate insurance policy, we will renew our nuclear deterrent based on the Trident missile system."
BBC defence correspondent Caroline Wyatt says that with only weeks to go before the defence review is unveiled, the bulk of the pain is thought likely to fall on the Royal Navy and the RAF.
If the cuts are major, they will be politically very hard for Defence Secretary Liam Fox - and his party - to defend, our correspondent adds.
In a leaked letter, Dr Fox has warned "draconian" cuts to defence spending would result in "grave consequences".
In an earlier meeting of the National Security Council, the RAF's Tornado force, the aircraft carriers and the size of the Army were discussed.
Defence analyst Jason Alderwick told the BBC: "The press has said it all - a navy the size of Portugal's.
"That is obviously a concern, that the Navy would lose the workhorses of their fleet to compensate for the fact those carriers are going through - which means a reduction in the navy's footprint in the world.
"They are heavily committed in many parts of the world, and something would have to give."