A man thought to have hoarding a "significant amount" of the toxic chemical DNP was arrested in an early morning raid in Harrow on Wednesday (February 26).
DNP has been linked to the deaths of five people last year. Sold legally as an industrial chemical, the substance is often found in banned supplements that claim to burn fat, and was originally used as in explosives during the Second World War.
The arrest marks the first of its kind in the UK and a significant step for authorities hoping to deter suppliers and those considering taking the substance in the form of dietary pills.
A spokesperson for the Food Standards Agency confirmed that chemicals discovered in the raid will have to go through a testing phase to confirm they are DNP before any charge or prosecution is borought. They said a quantity of steroids was also found.
The FSA regularly collaborates with the charity Beat, whose 2015 report The Costs of Eating Disorders estimated that 725,000 people in the UK are affected by an eating disorder, many of whom claim to use diet pills as a way to lose weight.
FSA chief Andy Morling said: "The FSA and its partners would like to use this as an opportunity to once again highlight the dangers of DNP.
"It is an industrial chemical and is not made to be consumed as a diet supplement."
Anyone affected by an eating disorder can call the Beat helpline on 0345 634 1414.