Bob Ainsworth has said there could be "major shifts" in defence spending.

In a keynote speech setting out government plans, the defence secretary also pledged that supporting British troops in Afghanistan would be his priority.

But he warned: "In the context of the defence review, we cannot exclude major shifts in the way we use our defence spending to refocus on our priorities. There will be tough choices ahead."

And he told Sky News that the capabilities of the Trident replacement may be reconsidered.

"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," he accepted.

Ainsworth's speech also acknowledged the pressures that tighter public spending rounds will bring.

"Despite continued real term increases, the pressures on the defence budget are well documented," he said.

"There are competing demands on the public purse. We will need to be better at spending the money we have, and more rigorous in prioritising what we spend it on."

The Gray review of defence procurement is to be published this autumn, and a defence green paper will follow early next year.

The Ministry of Defence will also have to balance competing requirements, Ainsworth noted in his speech.

"The need to maintain credibility in our primary role as the ultimate guarantor of our territorial integrity.

"And the ability to engage abroad at different levels of intensity preventing and resolving conflicts in order to protect our national security and our national interests."