Canada's police chiefs are calling on Ottawa to reject some of the key recommendations in a federal report on the legalization of marijuana, stating the proposals by former Liberal minister Anne McLellan will be impossible to enforce.

In a new discussion paper, the Canadian Association of Chiefs of Police (CACP) calls on Ottawa to "hold off on home grows" when it tables legislation in the spring to legalize marijuana for recreational purposes.

The top law-enforcement officers in the country agree that personal cultivation could eventually be allowed, but warn it should not happen at the same time as the recreational market is opened up to the private sector.

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CACP said police only have a limited ability to control in-home production, and that legalizing personal production would open the door to the "diversion to black markets" of marijuana. In addition, the association said personal production would run counter to the government's plans for a "highly regulated and controlled system," and would be "contrary to other measures to minimize child/youth exposure and access to cannabis products."

In a report released last December, a task force chaired by Ms. McLellan urged the government to allow Canadians to buy or carry 30 grams of marijuana for personal use, and to grow up to four plants at home.

Ms. McLellan did not put much emphasis in her report on the need to find ways to reduce the risks of impaired driving before the drug is legalized, stating the best solution was to give researchers additional time to develop proper detection tools.

"This is not going to be a new challenge that is created by legalization. Drug-impaired driving is a problem, or a challenge, in Canada today," Ms. McLellan said after the release of her task force's report.

However, Canada's police chiefs said drug-impaired driving "will become an even greater issue with legalization."

"We are very concerned that the prevalence of driving under the influence of drugs is not on Canadians' consciousness," the discussion paper said.

There is no recognized technology or exact limits to quantify marijuana impairment in Canada, which makes it hard for police to enforce existing laws. The police chiefs said the best way to tackle the issue at the moment is with trained and qualified Drug Recognition Experts, who can only receive field certification in the United States.

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"The CACP strongly recommends that governments increase investment in Drug Recognition Experts (DREs) and associated officer training to improve law enforcement's ability to detect and remove drug-impaired drivers from our streets," the discussion paper said.

CACP pointed out that it supports a majority of the recommendations put forward by Ms. McLellan, including the call for public-education campaigns on the dangers of marijuana and drug-impaired driving, strict labelling requirements, and a highly regulated production and distribution model.

Liberal MP Bill Blair, parliamentary secretary to the justice minister and the government point man on the marijuana file, said in an interview this week he is aware of the concerns of law-enforcement officials.

"It's part of an ongoing dialogue," said Mr. Blair, the former chief of the Toronto Police Service. "We're going to have to work very closely with the police leadership in this country and their perspectives are important to the discussion."

The legalization of marijuana for recreational purposes is about two years away. A senior federal official said last year that Ms. McLellan's report was well received inside the government and will have a large influence on the upcoming legislation to legalize marijuana, which will be tabled in Parliament in the spring of 2017.

The official explained that opening up the legal market will depend on the "readiness of the provinces," which will be in charge of regulating the wholesale distribution and retailing of cannabis. As such, the official said that implementing the new regime can be expected in 2019.