The former deputy prime minister Sir Nick Clegg has accused Theresa May of “ostrich-like sticking her head in the sand” over the reform of drug laws.

Clegg, who served alongside May when she was home secretary in the coalition government, said the main obstacle to much-needed reform was that politicians on the right were afraid of the “political hassle” of changing the law.

Speaking at a lunch hosted by the Global Commission on Drug Policy, of which he is a member, Clegg said: “I experienced coalition with the Conservatives, and I couldn’t find a sensible Conservative who didn’t privately entirely agree that the law is a complete ass and everything should be changed. They just didn’t think it was worth the political hassle.

“Our current prime minister is the exception. She is, genuinely and authentically, ostrich-like sticking her head in the sand. But most – [David] Cameron, [George] Osborne, [Oliver] Letwin, all those people – they don’t need persuading of the case, they just need persuading that it’s politically worth doing anything about it.”

The commission, set up in 2011, counts among its 25 commissioners the former UN secretary general Kofi Annan, a number of former senior officials from the UN, and former presidents of Nigeria, Switzerland, Poland, Chile and Colombia.

Clegg was speaking before the launch of the commission’s latest report, which calls on policymakers to change their perception of people who use drugs and the language they use to frame the debate about drug laws.

“Leaders must be bold when disputing perceptions about drugs which are not grounded in facts and which may be discriminatory towards people who use drugs, and stand their ground in the face of public opinion,” the report recommends.

“When political leaders choose to stoke fears about drugs and drug use in order to retain or intensify prohibition, they are indirectly causing serious hardship to some of their most vulnerable citizens.”

Helen Clark, the former prime minister of New Zealand, said the stigmatising language in the debate about drugs dehumanised people who suffered as a result of drug use. “I understand in this country [the UK] at the moment there is quite a spike in drug-induced deaths,” she said. “In my country that would lead to some action, but perhaps perceptions here are such that nothing has happened.”

Ruth Dreifuss, the former president of Switzerland, who is chair of the commission, said many of the negative perceptions towards drugs were enshrined in the wording of the UN conventions and declarations that form the basis of the international drugs control system. According to these documents, “there is a world drug problem, the substances are evil, the people are playing with their life and they are criminals,” Dreifuss said.

The commission is calling for a review of the use of the language used in these documents at the UN Commission on Narcotic Drugs ministerial segment in 2019.