Doctors in the House of Commons are to lead a campaign to change the law banning the medicinal use of cannabis, as a new all-party parliamentary group (APPG) forms to campaign for the issue.

Dan Poulter, a former health minister who still works part-time as a GP, said he had already signed up fellow Conservative Andrew Murrison, Labour’s Paul Williams, and Philippa Whitford of the Scottish National party – four of the Commons’ nine medical doctors.

On Thursday morning he was inviting colleagues to join an APPG on medicinal cannabis. He is to co-chair the group with Mike Penning, a former Tory justice minister who estimated they have the support of around 80 politicians.

The development comes after Alfie Dingley, a six-year-old boy with epilepsy, pushed medical cannabis on to the political agenda by taking a 300,000 signature petition with him to No 10. He had been effectively treated with cannabis oil in the Netherlands, but was denied it at home in the UK.

The Home Office is understood to be on the verge of finding a way for Alfie to continue his treatment. But another boy, Billy Caldwell, 12, has emerged in similar circumstances after his GP was reprimanded by officials for trying to write a cannabis prescription. Campaigners claim there are hundreds of other families facing the same problems.

“What these cases help to do bring into focus some of the absurdities about the law at the moment,” Poulter said. He said there is a general consensus among MPs that the ban on cannabis-derived drugs should be lifted, but the support of so many doctors in the Commons would give the campaign extra weight.

He and colleagues will look “in a rigorous scientific way” at evidence from countries that allow medicinal cannabis use, then make recommendations to the government on how the law could be changed.

Details of the plan were revealed on Tuesday by Crispin Blunt, the Conservative co-chair of the all party parliamentary group on drug policy reform, at a medical cannabis investors conference.

Legalisation of medical use of cannabis will be seen by many as a first step towards full legalisation. The plan comes as Canada, which allowed medicinal use in 2001, prepares for Justin Trudeau’s Liberal government to enact its manifesto pledge for a fully legal market.

But anyone hoping for a similar route to legalisation in the UK would be “drawing the wrong conclusions”, Penning said. “I would not be part of or even consider being a party of an APPG where [legalisation] was the end goal.”

A distinction needed to be drawn between the wider drug law debate and patients, such as Alfie Dingley, who are prescribed drugs, he said. “He doesn’t go into a bar in Amsterdam and have a spliff, it’s completely different and that’s why we’ve set up this group.”

Billy Caldwell’s mother, Charlotte Caldwell, said she had met Poulter and that she backed his plan. “This is not about one boy, this is about all of the children that desperately need this medicine,” she said.

“This is so important. This now is about every child in our country that’s suffering from this brutal condition. And now is the time to get these children their medicine, to give them a better quality of life.”