The Army faces the loss or merger of whole units as its regular strength is cut by 20,000, Defence Secretary Philip Hammond has warned.

Mr Hammond insisted he was not abandoning the regimental system, but said "difficult decisions" could not be avoided as the Army scaled back.

Addressing an audience of experts at the Royal United Services Institute in London, he said in future there would be greater use of part-time reserves and private contractors.

It would mean "thinking innovatively about how combat service support is provided" and "using more systematically the skills available in the reserve and from our contractors".

The changes would involve having to "rethink the way we deliver every aspect of military effect in order to maximise capability at the frontline", he said.

Mr Hammond said that Britain would also have to look more to its allies to provide military support while it concentrated on the frontline.

Under plans set out in the Strategic Defence and Security Review (SDSR), the Army is to be reduced from 102,000 soldiers to 82,000 by 2020.

Mr Hammond said: "A regular Army of 82,000 will have a different structure to one of 102,000. And some units inevitably will be lost or will merge."

He will add that "the history and the heritage" of some units deliver "tangible military benefits in the modern British Army".

"So there is no question of abandoning the regimental system. But that does not mean that we can avoid difficult decisions as the Army gets smaller. And in making those decisions, the military voice must prevail; ensuring that the Army remains the capable and agile force envisaged in the SDSR."