The party drug "miaow miaow" is expected to be formally banned today but dealers will still have a week's grace to hoard and sell it before facing arrest and prosecution.
The Lords are likely to approve making mephedrone a Class B drug after it was given the green light by MPs yesterday but the ban will not formally come in to effect until April 16.
The Government's drugs advisory body have already warned dealers are likely to stockpile the drug to sell it at greater profits when it becomes illegal.
The Advisory Council on the Misuse of Drugs (ACMD) also predicted users may buy large quantities while it is still legal.
Alan Johnson, the Home Secretary, announced plans to ban the drug last month after it was linked to a number of deaths.
The ACMD heard that up to 26 people may have died from its effects which Professor Les Iversen, the chairman of the body, said was extremely worrying.
A report by the ACMD also warned: "There are indications that criminal groups are becoming involved in the supply of mephedrone to the public in the UK.
"There are reports of some UK drug suppliers selling mephedrone in dance clubs and at street level either as well as, or instead of cocaine and MDMA, due to mephedrone's relatively low price high purity and easy availability."
It warned that in Guernsey, where importation of the drug is already banned, a street trade had already developed and was being linked with violence.
The report added: "It is reported that some users are planning to buy large quantities of mephedrone to 'stockpile' for future use and future sale should regulation be introduced."
As a Class B drug, those caught in possession will face a jail term of up to five years while dealers caught selling the drug will face 14 years.
By Tom Whitehead
April 8, 2010