Up to 4,480 serving soldiers in the Army will lose their jobs – including comrades of murdered soldier Lee Rigby in 2nd Battalion The Royal Regiment of Fusiliers.
The substancial cuts will be confirmed as the PM holds talks in Northern Ireland with other world leaders over whether Britain should arm Syrian rebels.
Commenting on this the Ministry of Defence (MoD) said the move was necessary to help balance the books but insisted operational capability would not be affected.
The government has said the Army “must live within its means.”
Defence Secretary Philip Hammond has confirmed there will be no further reductions in manpower in the next round of spending cuts.
Announcing the latest tranche of redundancies – originally thought to involve 5,300 soldiers – he said: “Today we have announced the third tranche of redundancies as we restructure the British Army to the size and configuration set out under the 2010 Strategic Defence and Security Review.
“It is with great regret that we have had to make redundancies to deliver the reduction in the size of the armed forces, but unfortunately they were unavoidable due to the size of the defence deficit that this Government inherited.
“Although smaller, our armed forces will be more flexible and agile to reflect the challenges of the future with the protection and equipment they need.
“They will continue to be the bedrock of our society and provide extremely rewarding and exciting careers for future recruits.”
But shadow defence secretary Jim Murphy said the redundancy notices issued today “represent not just broken promises but a failing strategy” to reform the British Army.
Personnel selected for redundancy would be told face to face by their commanding officer, the MoD said.
A spokeswoman said: “Tough decisions needed to be made to address the multibillion-pound deficit and bring the defence budget back into balance.
“This unfortunately included making some redundancies across the armed forces. However we can be clear that these reductions will not affect our operational capability.
“The end of combat operations in Afghanistan and the restructuring of our armed forces means they will be more reflective of the complex global situation and more adaptable to future challenges and threats.”