Theresa May and her aides attempted to delete sentences they didn't agree with from a report on drugs, the former deputy Prime Minister Nick Clegg has claimed.

The Liberal Democrat MP told the Guardian, the Home Secretary “didn’t like the conclusions” of a report released in 2014, which found no clear link between harsh drugs laws and illegal drug use.

The report, Drugs: International Comparators, said its fact-finding did not “observe any obvious relationship between the toughness of a country’s enforcement against drug possession, and levels of drug use”, sparking requests for further talks regarding decriminalisation.

The conclusions were derived from Portuguese data, where the report found there had been “considerable” health improvements since the country began to treat drug possession as a health rather than a criminal issue.

Mr Clegg, who sits on the Global Commission on Drugs Policy, was one of a number prominent figures who signed an open letter to the UN Secretary General, Ban Ki-moon, urging the UN to review its 20-year campaign to rid the world of illegal drugs, ahead of a special summit in New York next week.

In 1998, the UN formally committed itself to abolishing illegal drugs worldwide within 10 years. Recently, however, a reactionary movement has emerged which holds that the so-called War on Drugs, rather than helping the world, has created vast and powerful criminal networks and endless cycles of violence.

World's 10 deadliest street drugs Show all 10 1 /10 World's 10 deadliest street drugs World's 10 deadliest street drugs Whoonga Whoonga is a combination of antiretroviral drugs, used to treat HIV, and various cutting agents such as detergents and poisons. The drug is widely available in South Africa due to South Africa’s high rate of HIV sufferers, and is believed to be popular due to how cheap it is when compared to prescribed antiretrovirals. The drug is highly addictive and can cause major health issues such as internal bleeding, stomach ulcers and ultimately death Getty World's 10 deadliest street drugs Scopolamine Scopolamine is a derivative from the nightshade plant found in the Northern Indian region of South America (Ecuador, Colombia and Venezuela). It is generally found in a refined powder form, but can also be found as a tea. The drug is more often used by criminals due its high toxicity level (one gram is believed to be able to kill up to 20 people) making it a strong poison. However, it is also believed that the drug is blown into the faces of unexpecting victims, later causing them to lose all sense of self-control and becoming incapable of forming memories during the time they are under the influence of the drug. This tactic has reportedly been used by gangs in Colombia where there have been reports of people using scopolamine as way to convince victims to rob their own homes World's 10 deadliest street drugs Heroin Founded in 1874 by C. R. Alder Wright, heroin is one of the world’s oldest drugs. Originally it was prescribed as a strong painkiller used to treat chronic pain and physical trauma. However in 1971 it was made illegal under the Misuse of Drugs Act. Since then it has become one of the most destructive substances in the world, tearing apart communities and destroying families. The side effects of heroin include inflammation of the gums, cold sweats, a weak immune system, muscular weakness and insomnia. It can also damage blood vessels which can later cause gangrene if left untreated World's 10 deadliest street drugs Crack cocaine Crack cocaine first came about in the 1980’s when cocaine became a widespread commodity within the drug trafficking world. Originally cocaine would have attracted a high price tag due to its rarity and difficulty to produce, but once it became more widespread the price dropped significantly. This resulted in drug dealers forming their cocaine into rock like shapes by using baking soda as a way of distilling the powder down into rock form. People were doing this because it allowed for them to sell cocaine at a lower quantity and to a higher number of people. The side effects of crack cocaine include liver, kidney and lung damage, as well as permanent damage to blood vessels, which can often lead to heart attacks, strokes, and ultimately death World's 10 deadliest street drugs Crystal meth Not just famous because of a certain Walter H White, but also because it is one of the most destructive drugs in the world. First developed in 1887, it became widely used during the Second World War when both sides would give it to their troops to keep them awake. It is also believed that the Japanese gave it to their Kamikaze pilots before their suicide missions. After the war crystal meth was prescribed as a diet aid and remained legal until the 1970s. Since then it has fallen into the hands of Mexican gangs and has become a worldwide phenomenon, spreading throughout Europe and Asia. The effects of crystal meth are devastating. In the short-term users will become sleep depraved and anxious, and in the long-term it will cause their flesh to sink, as well as brain damage and damage of the blood vessels World's 10 deadliest street drugs AH-7921 AH-7921 is a synthetic opioid that was previously available to legally purchase online from vendors until it became a Class A in January 2015. The drug is believed to have 80% of the potency of morphine, and became known as the ‘legal heroin’. While there has only been one death related to AH-7921 in the UK, it is believed to be highly dangerous and capable of causing respiratory arrest and gangrene World's 10 deadliest street drugs Flakka Flakka is a stimulant with a similar chemical make-up to the amphetamine-like drug found in bath salts. While the drug was originally marketed as a legal high alternative to ecstasy, the effects are significantly different. The user will feel an elevated heart rate, enhanced emotions, and, if enough is digested, strong hallucinations. The drug can cause permanent psychological damage due to it affecting the mood regulating neurons that keep the mind’s serotonin and dopamine in check, as well as possibly causing heart failure World's 10 deadliest street drugs Bath salts Bath salts are a synthetic crystalline drug that is prevalent in the US. While they may sound harmless, they certainly aren’t the sort of salts you drop into a warm bath when having a relaxing night in, they are most similar to mephedrone, and have recently been featured throughout social media due to the ‘zombification’ of its. The name comes from the fact that the drug was originally sold online, and widely disguised as bath salts. The side effects include unusual psychiatric behaviour, psychosis, panic attacks and violent behaviour, as well as the possibility of a heart attack and an elevated body temperature World's 10 deadliest street drugs Purple Drank One of the more unusual drugs around at the moment, purple drank was popularised in 90s hip hop culture, with the likes of Jay Z and Big Moe all mentioning it in their songs. It is a concoction of soda water, sweets and cold medicine, and is drunk due to cold medicines high codeine content, which gives the user a woozy feeling. However it can also cause respiratory issues and heart failure World's 10 deadliest street drugs Krokodil Krokodil is Russia’s secret addiction. It is believed that over one million Russians are addicted to the drug. Users of krokodil are attracted to the drug due to its low price; it is sold at £20 a gram while heroin is sold for £60. However, krokodil is considered more dangerous than heroin because it is often homemade, with ingredients including painkillers, iodine, lighter fluid and industrial cleaning agents. This chemical make-up makes the drug highly dangerous and likely to cause gangrene, and eventually rotting of the flesh

The summit, set to open on 19 April, is likely to be a landmark meeting as it appears it will offer a platform to those who question the current drugs policy for the first time.

The session was brought forward two years at the request of Mexico, Guatemala and Colombia, three countries on the front lines of the narcotics conflict.

Mr Clegg told the Guardian said the original draft of the 2014 report had been subject to an “endless wrangle between Lib Dem ministers and Theresa May about the fullness of what would be published”, arguing that there would be no change whatsoever as long as she led the Home Office.

A Home Office spokesperson said: "The UK's approach on drugs remains clear: we must prevent drug use in our communities and help dependent individuals to recover, while ensuring our drugs laws are enforced.

"We have seen a reduction in drug misuse amongst adults and young people over the last 10 years and more people are recovering from their dependency now than in 2009/10. Decriminalising drugs would not eliminate the crime committed by their illicit trade, nor would it address the harms and destruction associated with drug dependence.

"The International Comparators Study does not say there is no link or impact between tough penalties and drug use. It makes clear that approaches to drugs legislation and enforcement of drugs possession are only one element of a complex set of factors that affect drugs use, including prevention, treatment and wider social and cultural factors.”

The move marks one of Mr Clegg’s first campaigns since stepping down as leader of the Liberal Democrats after their general election disaster.

Nick Clegg on election defeat: 'The first thing I do is reach for a cigarette'

Last year Mr Clegg launched a campaign to persuade EU leaders to back global reform of drug laws, warning that the current punitive approach has failed to curb the multibillion trade in illicit substances and has criminalised millions of young people.