Mephedrone has been legal and widely available
Airport officials have made their first seizure of a suspected mephedrone-like substance since its importation was banned, the UK Border Agency has said.
Officers at Stansted Airport, Essex, seized 103kg in five packets labelled as methylone, a compound similar to the drug ministers hope to ban by April 16.
Tests will now take place on the substance, which had been bound for London and arrived from Mumbai, India.
No charges will be brought because the packets were sent before the ban.
Home Secretary Alan Johnson banned the importation of mephedrone and similar derivatives on Monday.
The move came after the Advisory Council on the Misuse of Drugs recommended they be declared illegal and categorised as Class B drugs, the same group as cannabis and amphetamines.
The drug is said to produce similar effects to the latter, as well as cocaine and Ecstasy.
Ministers say it has been linked to a number of deaths and can caused hallucinations, blood circulation problems, paranoia, fits and delusions.
The UK Border Agency said the exporter made no attempt to conceal the contents of the seized packages.
They were clearly marked "Methylaminopropan-1-one" - known commonly as methylone - one of the banned compounds.
The agency's Brodie Clark praised officers for making the "substantial seizure".
"The UK Border Agency is determined to stop harmful drugs reaching our streets. They devastate communities and destroy lives," he added.
Both mephedrone and methylone are derived from the same chemical, found in the khat plant.
The charity Drugscope suggests methylone produces effects similar to Ecstasy.