VANCOUVER -- Authorities have intercepted almost 18,000 litres of a controlled substance that's "essential" to the production of date-rape drugs, the Canada Border Services Agency said Thursday.

Border officers from the CBSA's Metro Vancouver Marine Operations found the substance while examining a "container of interest" back in February.

Testing has since determined it to be gamma-butyrolactone, or GBL, a Class A precursor under the Controlled Drug and Substances Act, the CBSA said.

According to a report by the World Health Organization, GBL can be "converted into GHB (date-rape drugs) in plain tap water without additional adjustments."

Altogether, officers seized about 17,853 litres of GBL, which the CBSA estimates has a street value of more than $1.8 million. Pictures provided by authorities show dozens of seized drums wrapped in plastic.

The CBSA said the investigation into the importers is ongoing.