He said the possibility of "social disorder" following Brexit is becoming a greater "worry", adding the Armed Forces "would be very, very reluctant to become involved".
"I don't see that it's (social disruption) likely," General Lord Richards said.
"But if one or two of the doom-mongers are to be believed and the scale of social disorder, say in the failure to come out of the EU...you only need a couple-of-hundred-thousand - a small proportion to act up and the Police could have a problem.
"But I think that is one end of the spectrum, I don't see that is likely."
General Lord Richards added he has "no doubt" that the Armed Forces have already been doing some planning ahead of a no-deal scenario.
Any reservists who are mobilised for full-time service will be sent a 'call out' notice.
In most cases, personnel get 28 days' notice upon receiving a 'call out', however this may decrease in urgent situations.
When asked how long could the Armed Forces sustain working alongside civil authorities, General Lord Richards said: "The Armed Forces are too small, they are well below the level they're meant to be at.
"That's one reason why the reserves have been called out...and I think we could go on doing it for a while."