The campaign spearheaded by Russell Brand, Sting and Sir Richard Branson has also received backing from the Prison Governors Association and the National Black Police Association.
They want the Prime Minister to act, so drug possession is no longer a crime.
The letter says the use of legal sanctions for the possession of drugs in the UK has led to the "unnecessary criminalisation" of more than 1.5 million people in the last 15 years.
Marolin Watson from Christian drugs charity Hope UK told Premier's News Hour why she doesn't think decriminalisation is the answer either.
The letter goes on to state that evidence from Australia, the Czech Republic and Portugal shows that health problems linked to drugs are "dramatically" reduced when users are given medical support and advice rather than being prosecuted.
Thousands of activists have been taking to the streets in more than 80 cities to fight what they're calling "harmful drug laws", which campaigners say have been the cause of health crises, instability and mass incarceration around the world.
A demonstration was held in Parliament Square in London, on Thursday morning, with other actions planned in New York, Paris, Warsaw, Mexico City, Kathmandu, Rome, Phnom Penh, Tbilisi, Kuala Lumpur, Moscow and more than 70 other cities.
The actions include peaceful demonstrations, street performances, public meetings and workshops, social media campaigns and advertisements on public transportation and billboards.
The events have been organised to coincide with United Nations' International Day against drug abuse and illicit trafficking.
Organisers say the UN's anti-drugs day is being used by many governments to justify violent crackdowns and to promote harsh punishments.
They want the "Support. Don't Punish: Global Day of Action" to reclaim this day and promote a more effective and humane approach to drugs that is based on public health and human rights.