The Archbishop of York warned that defence cuts will compromise Britain's safety as he delivered his Christmas message to soldiers serving overseas.
In an interview with British Forces Broadcasting Service, Dr John Sentamu said severe cuts to the armed forces would "risk the safety of the nation and the peace of the world".
He also said plans to replace thousands of full-time soldiers with reservists was a bad idea as a "very well trained, professional service" needed to be maintained.
The Archbishop said: "My greatest anxiety... is that these defence cuts need to be done with far, far greater sensitivity because we live still in a world that is very fragile and there are people out there still, wanting to do harm to... many, many people.
"To replace professionally trained, full time serving soldiers with part-timers, I'm afraid, for me, I don't think that can be the backbone of the British army."
Describing them as "the bravest of the brave", the Archbishop paid tribute to British forces' role as peacekeepers in Afghanistan.
He added that he continued to pray daily for the troops and that it was important that the nation remembered the sacrifices that were being made.
The Archbishop has close links with the Yorkshire Regiment who lost 10 soldiers this year serving in Afghanistan.
Defence Secretary Philip Hammond has warned the Army will have to lose 20,000 regular soldiers by 2020.
The number of full-time soldiers is set to fall from 102,000 to 82,000, while reservists will double to 30,000.