The US Department of Justice has officially reclassified how it views cannabis, a step marking a historic shift in the country's drug policy.

The move changes cannabis, also known as marijuana, from a Schedule I narcotic to a Schedule III drug - placing it in the same category as Tylenol with codeine.

President Donald Trump signed an executive order last year directing his administration to begin the reclassification process, in order to increase access and research of the drug.

Marijuana remains illegal at the federal level. Despite the federal ban, most US states have legalised the medical or recreational use of marijuana, and many have shops where it can be purchased legally.

On Thursday, Acting Attorney General Todd Blanche ordered the official reclassification of products containing marijuana that are regulated by the federal Food and Drug Administration and products that have received a state medical marijuana license.

The move had been expected to happen since December, when Trump first ordered the reclassification process to begin.

Once the rule change is published in the Federal Register, it has 30 days until it takes effect. During that time, it can be legally challenged - which is expected - and be blocked from being implemented for months or even years.

The Drug Enforcement Agency (DEA) will hold a hearing on the change in late June.

Five days ago, Trump signed an executive order intended to boost access to psychedelic drugs for medical treatment.