The stars are backing the Support, Don't Punish campaign by U.K. charity Release to change the way lawmakers deal with drug users.
The letter, signed by 90 celebrities, lawyers, and health experts, will be sent to British Prime Minister David Cameron and claims 1.5 million citizens have been unnecessarily convicted on drug possession charges in the last 15 years.
The charity's bosses want new rules to help addicts rather than treat them like criminals.
As part of the drive, a protest has been scheduled to take place in Parliament Square in London on Thursday (26Jun14).
Brand, a former heroin addict, has long campaigned for the decriminalisation of drugs, and he was a speaker at a United Nations panel on the subject in Vienna, Austria in March (14).