Sir Malcolm Rifkind has said that security chiefs speaking to MPs in public today was "an historic occasion" that will be repeated.
The Chair of the Intelligence and Security Committee added that he believed those involved were glad of the chance to speak publicly.
He said: "Many of them have been upset at the accusations that they have been doing something improper or doing something unreasonable to their fellow citizens.
"I suspect they were pleased to have the opportunity in public to speak about the work their own staff do and how important it is to the national interest."
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."