About 23,000 pills were taken during a raid in Waterlooville, Hampshire, by officers working for the Medicines and Healthcare products Regulatory Agency (MHRA).
MHRA is responsible for regulating all UK medicines and medical devices.
Some of the drugs were marked "herbal", but actually contained the banned pharmaceutical ingredient sibutramine.
Sibutramine was licensed as the medicine Reductil until 2010, when it was withdrawn across Europe and the US because of an increased risk of heart attacks and strokes associated with the use of the drug, the MHRA said.
The agency's head of operations, Danny Lee-Frost, said: "The action we have taken to seize these products demonstrates our total commitment to tackling this dangerous and illegal trade and to protecting patients.
"Criminals involved in the illegal supply of medical products aren't interested in your health - they are only interested in your money."
He said that the pills "may contain dangerous ingredients" and warned that the "consequences (of taking them) could be devastating".
Mr Lee-Frost added: "The internet offers access to a vast number of websites offering a wide range of products marketed as 'slimming pills' or 'diet pills'. Many make attractive claims and offer quick-fix solutions. Be aware that 'natural' doesn't always mean safe."
Unlicensed slimming pills with an estimated value of £30,000 have been seized by enforcement officers.