The defence secretary has unveiled plans for a dramatic overhaul of Britain's armed forces and defence systems.

Publishing the defence white paper, Geoff Hoon signalled a shift in emphasis in the structure of British armed forces.

Hoon said conflict is becoming "increasingly sophisticated" and reliant on high-tech precision weaponry.

British forces must be able to "act quickly, accurately and decisively", the defence secretary said.

"The shadow of the Cold War, which has shaped our armed forces for two generations, may well have receded. And the threat of a large-scale, conventional military attack on Europe may seem remote as a result," Hoon told MPs.

"But new threats are emerging. We must respond to today's strategic environment and prepare for tomorrow's."

Hoon said the proliferation of weapons of mass destruction and the threat posed by international terrorism - coupled with the consequences of failed or failing states - present the UK "with a very real and immediate challenge".

The white paper calls for more rapidly deployable units able to fight small wars and combat terror.

It also confirmed that new technology will be vital if Britain is to keep up to date with global shifts in warfare.

"This is a changing world and we must adapt if our armed forces are to stay ahead of potential adversaries," said Hoon.

"We must exploit new and emerging technologies, and we must be prepared to make tough decisions to ensure that our armed forces are able to carry out the difficult tasks that we ask of them."

Hoon said UK forces should be able to commit to several operations at any given time.

"The need for multiple, concurrent small to medium sized operations will be the most significant factor in force planning," Hoon told MPs.

But plans to rationalise the structure of the forces by merging regiments will spark anger.

The defence secretary confirmed the size of the UK's heavy armed forces will be reduced.

A new light brigade will be established, reducing the number of armoured brigades from three to two.

Further plans to overhaul the structure of the armed forces will be unveiled next year, he told MPs.

Hoon announced that the structure of each of the armed services would also "need to evolve to optimise joint operations".

Whilst Britain would only enter into major combat operations alongside America, Hoon said existing capacity levels should be maintained.

"We must also retain the capacity to reconfigure our forces at longer notice to undertake the less frequent, but more demanding large-scale operation of the type we saw in Iraq," he added.

"Expeditionary operations on this scale can only effectively be conduced if US forces are engaged."

Shadow defence secretary Nicholas Soames said Hoon must consult widely before restructuring the armed forces.

"There is a great deal of work to be done," he said.

"The greatest possible thought and care needs to be taken in implementing these future programmes and the profound changes they foretell if we are not to imbalance something that is truly excellent."

And he went on to warn that investment in new technology should not be a substitute for full-time, fully-trained service personnel.

"Whilst we recognise the demands and the need to embrace wholeheartedly the new technologies, we remain deeply concerned about the financial crisis in the Ministry of Defence," he added.