The decision to close Kingston Penitentiary and absorb the hundreds of maximum security inmates into other facilities was premature and poorly thought out, Canada's correctional workers' union president said on Friday.

Jason Godin, Ontario president of the Union of Canadian Correctional Workers, says he has grave doubts about shutting down Kingston penitentiary and the psychiatric institution housed inside, called the Regional Treatment Centre.

"There has been very little thought put into this decision. We certainly have a lot of concerns with where we are going with the future of federal corrections in Canada," Godin told CTV Canada AM on Friday.

The closure means more than 400 maximum security inmates will need to be transferred to other facilities, where their daily requirements will need to be learned by new corrections officers and may not be met, Godin said.

Godin estimates that 20 per cent of Kingston Penitentiary's inmates are mentally ill. Among the population are also some of the most dangerous criminals in Canada, and includes such high-profile members as Paul Bernardo and Russell Williams.

"We are now going to take 450 maximum security inmates and try to absorb them into other inmate populations," Godin said. "Because the population is so unique at Kingston penitentiary it is virtually impossible to absorb that number of maximum security inmates into other populations. They are just not compatible."

Public Safety Minister Vic Toews announced the decision to close Kingston Pen, as well as the medium-security Leclerc Institution in Laval, Quebec, on Thursday.

The minister said the facilities are housed in aging buildings with old infrastructures that don't meet the needs of today's prisoners.

Kingston Mayor Mark Gerretsen said was caught off guard by the decision to close the historic facility, which he first learned about on Facebook rather than from the federal government.

"It is a facility that has been in operation for 177 years plus, one of the oldest operating maximum security facilities not only in Canada but in the world," Gerretsen told CTV News Channel. "We were surprised, but perhaps we were surprised most by the manner in which we learned about the information."

The closures will save $120 million a year, the minister said, adding the government has no intention of building new prisons. Instead, prisoners and correctional officers will be moved to other locations.

Godin said Kingston penitentiary has received hundreds of millions in upgrades and retrofitting, bringing up to the standard of a "functioning prison for a very distinct population group."

He added that the Union of Canadian Correctional Officers had little choice but to trust Toews that those employees would be absorbed into other Kingston-area institutions.

"This decision has been made very prematurely. There is no plan on how we are going to manage those offenders in other populations. We don't have existing infrastructure, nor are we going to have infrastructure in the future that is going to be able to manage those types of inmate populations," Godin said.

The closures come as part of a round of budget cuts that will see the Conservatives cut spending by about $5.2 billion over the next three years.

Opened in 1835, the Kingston Penitentiary is one of the oldest prisons in continuous use in the world. It is one of nine prisons in the Kingston area and houses between 350 and 400 inmates, plus another 130 inmates at the psychiatric treatment centre inside the prison.

Some of Canada's most notorious convicted murderers are housed there, including Paul Bernardo, Russell Williams and Mohammad Shafia.

The Leclerc Institution opened in 1961 and houses 481 inmates including a number of organized crime figures.

Corrections Canada is the third largest employer in the Kingston Region with more than 4,000 positions. Those include 465 at Kingston Penitentiary, 178 employees at the Regional Treatment Centre, and 358 people employed at Leclerc Institution.