The UK's top military officer has indicated a major overhaul of Britain's armed forces to respond to the demands of combating international terrorism.
The UK military looks set for change
Chief of the Defence Staff Sir Michael Walker signalled cuts in warships, aircraft and heavy armour.
The general - speaking ahead of Thursday's Defence White Paper - warned of "tough choices" ahead.
He said the plans had the backing of military top brass and were not being driven by politicians and accountants.
But Conservative defence spokesman Keith Simpson warned against cutting troop numbers.
"With the Army already under strength, and committed from Northern Ireland to Iraq and relying heavily on reservists, it would be highly irresponsible to cut the strength of the Armed Forces further," he said.
In a speech to the Royal United Services Institute, Sir Michael cautioned that there must be "no change for the sake of change".
He added: "But this White Paper is about building 21st century armed forces."
Changes were being driven by the need for a more "flexible and agile" armed forces to counter the spread of weapons of mass destruction and to tackle international terrorism.
"Counter-terrorism and counter proliferation operations in particular will require rapidly deployable forces able to respond swiftly to intelligence and achieve precise effects across the world," he said.
"This places a premium on the agility, deployability and sustainability of our forces."
With the Type 45 destroyer and two planned new aircraft carriers coming into service some of the Royal Navy's older warships would no longer be required - giving space for "some adjustments" within the existing fleet.
And new technology including the latest precision missiles would allow the RAF to achieve its military objectives while deploying fewer aircraft.
In the army, a new generation medium weight armoured vehicle would "inevitably reduce our requirement for heavy armoured fighting vehicles and
"Significant" amounts of cash would be invested so that weapons such as unmanned aerial drones could help spot and attack "targets of opportunity".
"It is inevitable that this will mean change - adapting to the changing strategic environment will require difficult choices to be made," he said.
"It would be quite wrong for us to retain systems, within a finite budget, which we know are no longer effective."
The restructuring, said Sir Michael, would enable Britain to mount "limited national operations" unilaterally or take the lead in small to medium operations in international conflict.
Mr Hoon said British troops could deploy anywhere in the world
UK forces would also retain the capacity to undertake large-scale operations but the "most demanding expeditionary operations" could only be "plausibly" mounted if the US was involved.
"Consequently, our Armed Forces will need to be interoperable with US command
and control structures, maintain the US operational tempo and provide those
capabilities that deliver the greatest impact when operating alongside the US,"
Defence Secretary Geoff Hoon told BBC News that Britain required "smaller forces able to move at very short notice to perhaps anywhere in the world".
"Our armed forces have to go through the kind of change that modern businesses have gone through relying on technology to deliver effect."
He also stressed the importance of British forces being able to operate alongside the Americans - something which he acknowledged was both "technologically challenging and financially expensive".
Mr Hoon issued a warning to European allies against any move that would polarise relations with the US.
"In some places, a parody has been developing of America which all but demonises its power and its policies, and seeks to put all of the ills of the world at its door," he said.
"The dangerous consequence of this is that it can feed misunderstanding and encourage isolationist tendencies on both sides of the Atlantic."
It was important both to Europe and to the US that there continued to be an "effective and sustainable transatlantic alliance", preferably via Nato.