Seizures of cheap substitutes for traditional drugs known as “bath salts” have surged more than sixfold year-on-year in the first four months of this year, as traffickers tried to smuggle the synthetic drug from the mainland to the United States through the city.

According to the latest figures obtained by the Post, customs officers netted 147kg of “bath salts” – properly known as synthetic cathinone – between January and April this year and arrested one person. This compares to 23kg seized in the same period last year with no arrests.

The haul in the first four months accounted for more than 77 per cent of “bath salt” seizures in the whole of 2016.

A 59kg haul in March marked the biggest case this year. The drugs were concealed in a number of air parcels destined for the US, where the drugs became popular in 2011.

The surge came after the Security Bureau warned about new types of drugs coming into the city over the past year. But the authorities stressed the seized drugs were not intended for local use and saw no such trend developing.