Former Defence Secretary Sir Malcolm Rifkind said there is a "serious question mark" over the Government's plan to plug the gap left by Army cuts by increasing the numbers of part-time soldiers.
The Conservatives have pledged to cut the size of the Army by 20,000 while doubling the size of the Army Reserves to 30,000 by 2018.
Former Tory cabinet minister Rifkind told the Telegraph: "The plan is very ambitious. There is a serious question mark about whether it's deliverable. I can certainly see it as being very challenging."