The British army will rely more on reserve forces, contractors and support from its allies as its full-time force is slimmed down by a fifth, Defence Secretary Philip Hammond said Thursday.

Entire units will be axed or merged with others as the army is shrunk from 102,000 soldiers to 82,000 by 2020 under a tough programme of cuts that forms part of Prime Minister David Cameron's austerity drive.

Hammond said he was "planning in detail the shift to a smaller army with its focus moving from campaigns to contingency", following wars in Afghanistan and Iraq and what he said had become an "overheated defence programme".

Speaking at the Royal United Services Institute in London, a defence think-tank, he added: "I am also determined that we rethink the way we deliver every aspect of military effect in order to maximise capability at the front line."

That meant "using more systematically the skills available in the reserve and from our contractors", Hammond said, in changes that stem from a major defence review published in 2010.

An extra 1.8 billion pounds ($2.9 billion, 2.2 billion euros) is to be spent on reserve forces over 10 years.

"The future reserves must be structured to provide, as they do today, some niche specialist capabilities that aren't cost-effective to maintain on a full-time basis -- for example in areas of cyber, medical, or intelligence," Hammond added.

The overhaul will also involve "working closely with partners to operate logistics more rationally through alliance structures (and) looking, sometimes, to others to provide the tail, where Britain is providing the teeth," Hammond said.

The British defence budget for this year is 34.4 billion pounds.

Hammond said last month he had fixed a 38 billion pounds so-called "black hole" of unfunded spending commitments in his ministry's budget.

Britain has around 9,500 troops serving with the NATO force in Afghanistan, but Cameron announced in July that the deployment would be reduced by 500 to around 9,000 this year.

Along with the rest of the NATO coalition, Britain is due to end combat operations in Afghanistan by late 2014.

Britain was also at the forefront of international efforts to support Libyan rebels who eventually ousted dictator Moamer Kadhafi's regime in 2011, launching United Nations-mandated military action with France and the US before NATO took over.