The Ontario government is coming under fire for delays in the delivery of legal cannabis in the province, with critics suggesting consumers could revert back to the black market.

Just over a week after recreational marijuana became legal, the provincially run Ontario Cannabis Store website is warning consumers that delivery times for their orders may be longer than expected due to "extremely high demand" and labour action at Canada Post.

Unionized Canada Post employees began rotating strikes this week and nearly 9,000 members walked off the job in the Toronto region on Tuesday.

The Toronto-area members of the Canadian Union of Postal Workers returned to work at midnight Thursday but the work stoppage at two facilities forced delays in shipments of tens of thousands of letters and parcels across the country.

Finance Minister Vic Fedeli said earlier this week that the government had a backup plan to ensure timely deliveries of cannabis, but said Thursday that the plan would only be implemented in the case of a general strike. He again declined to give details of the plan.

Opposition parties, meanwhile, suggested the difficulties in accessing legal recreational cannabis could encourage some users to turn to illegal suppliers.