Toxicology tests have reportedly found that two teenagers who were thought to have died as a result of taking mephedrone – leading to a successful campaign to ban it – had not taken the drug.

The deaths in March of Louis Wainwright, 18, and Nicholas Smith, 19, contributed to a furore over mephedrone, which was at the time legal and being sold over the internet.

The Labour government banned the legal high in the final days of the last parliament, sacrificing other legislation in order to push through an amendment that made the synthetic substance a class B drug.

Today the BBC reported that toxicology tests had revealed there were no traces of mephedrone in the blood of the two Scunthorpe teenagers. It said further tests were being conducted to try to establish what, if any, substances the two had taken.

Mephedrone – which prior to the ban was marketed as plant food with names such as Bubbles and M-Cat – is a synthetic imitation of cathinone, a compound found in the plant khat.

Humberside police prompted widespread concern in March when they held a press conference over the teenagers' deaths, announcing there was "information to suggest these deaths are linked to M-Cat". The force advised anyone who had taken the drug to attend a hospital "as a matter or urgency".

Drug campaigners, teachers, doctors and leading politicians called for an immediate ban on the drug, which ministers made a priority.

The force last night declined to comment on the toxicology tests, saying the final report was incomplete. "The findings of the report, once completed, will be forwarded to the coroner and may be discussed at any inquest and will not be disclosed without the authority of HM Coroner," it said in a statement. North-east Lincolnshire coroner's court has refused to comment.

Professor David Nutt, whom the government sacked last year as chairman of the Advisory Council on the Misuse of Drugs (ACMD) after he questioned government drugs policy, said the Scunthorpe deaths had been a tipping point in the debate over the drug.

He called for an inquiry into Humberside police for publicly speculating over the deaths. "The temperature was rising a bit, but the deaths got it boiling over," he said. "You can argue if that hadn't happened the previous government wouldn't have been bounced into this response. If these reports are true, the government's rush to ban mephedrone never had any serious scientific credibility – it looks much more like a decision based on a short-term electoral calculation."

He added: "This news demonstrates why it's so important to base drug classification on the evidence, not fear and why the police, media and politicians, should only make public pronouncements once the facts are clear."

The former home secretary, Alan Johnson, who repeatedly clashed with Nutt, responded that the decision had been taken after advice from ACMD, which was being led by a new chairman. "We took advice from the ACMD," he said in a statement. "The advice wasn't based on any deaths that had occurred in recent times, but on the prevalence and availability of the drug and its potential harm."

Professor Roumen Sedefov, a leading scientist who monitors new drugs for the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA), was mandated this week to begin a risk assessment of mephedrone.

He said he was aware of only one fatality in the world – that of a woman in Sweden – where the drug had been definitively identified as the cause of death.

Referring to data provided to him by the UK government, he said officials had identified 26 cases where mephedrone may have been a contributing cause of death. There had been definitive results in three cases, two of which showed that the drug, though present in the body, had not contributed to the death. The third death was put down to "adverse affects" of methadone and mephedrone, he said.

Methadone is a heroin substitute linked to thousands of deaths.