The defence secretary has refused to rule out a cut in troop numbers after the upcoming strategic defence review.
Liam Fox said the "the shape and size of our armed forces" had to change over time "in the light of the threats the country faces".
But he said the UK's nuclear deterrent would not be affected by the review.
It has been announced that chief of the defence staff, Air Chief Marshal Sir Jock Stirrup, will quit in the autumn after the review, before his term ends.
Sir Jock has been chief of the defence staff since 2006 and was asked to extend his term by the previous government. Top Ministry of Defence civil servant Sir Bill Jeffrey will also go.
The BBC's Caroline Wyatt said the move would be seen as the new government's bid to draw a line under past failings.
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