Canada's prison system is stretched to the breaking point and any sudden influx of new inmates would be "dangerous," the federal correctional investigator says.

Howard Sapers's stark warning Tuesday comes amid concern that the Harper government's tough-on-crime agenda could swamp already-strained prisons.

Public Safety Minister Peter Van Loan agreed policy changes will drive "a need for additional capacity" in custody. "It's undoubtedly the case that we're going to be building new prisons in the future," he said in an interview.

But that need has more to do with crumbling buildings and overall population growth than Conservative plans to lock up more people, he said.

For now, Van Loan said, he is confident that any increase will be gradual enough for the corrections service to handle.

Sapers said the country's 58 institutions are barely managing about 13,500 inmates. Another 8,000 prisoners are under varying degrees of supervision on the outside.

The majority, including sex offenders, never fully complete rehabilitation programs because of long waiting lists and frequent transfers, he said.

Adding to the pressure is a spike in the number of mentally unstable offenders in the last 10 years following the closure of residential institutions without the creation of adequate community supports.

Adding more inmates could tip the scales toward disaster, Sapers said.

"It would be dangerous to further burden the corrections system federally without additional capacity — and dangerous in the sense of both institutional violence and the risk of reoffence upon release," he said after appearing before the Commons public safety committee.

"We know historically that the more repressive conditions become inside institutions, the more dangerous conditions become — and that's for inmates and staff.

"The system right now is working at its capacity. And if we find the Correctional Service of Canada faced with a rapid influx of inmates without additional capacity — program capacity, accommodation capacity, human resource capacity — then those are the dangers that I foresee."

Conservative legislation to set mandatory minimum sentences for various drug crimes, along with plans to nix extra credit for time served in overcrowded remand centres prior to trial, could swell prison ranks, critics warn.