Conservative politicians who have admitted taking drugs have been branded hypocrites by campaigners taking part in a 60-mile march on parliament to demand the liberalisation of Britain’s drug laws.

The marchers from the Anyone’s Child group are walking along the Thames Valley and into London to meet MPs at Westminster on Tuesday.

Rose Humphries, the mother of two young men killed by heroin, said she hoped revelations that the former Tory leadership candidates Michael Gove and Andrea Leadsom took cocaine and cannabis respectively many years ago would help change both political and public attitudes towards legalising drugs.

Despite their sons’ deaths from heroin abuse, Humphries and her husband, Jeremy, have joined the movement agitating for the legalisation and state regulation of all recreational drugs.

Humphries said: “On hearing that Michael Gove took cocaine, while his government [has since] continued the failed policy of drug prohibition I thought to myself – ‘what hypocrites’.

“But then if it starts to make people out there think about the current situation where every recreational drug is banned then maybe that revelation was a good thing. Because it’s not just about changing politicians’ minds but also that of the public.”

The couple’s younger son, Roland, died aged 23 in 2003. Eleven years later Jake, 37, also died from heroin abuse. Yet the couple, who are in their early 70s, are strong advocates for legalisation.

“It’s so obvious that after 50-plus years the policy of prohibition doesn’t work; a policy that costs lives,” Jeremy Humphries said. “In relation to Roland he was with other people when he overdosed and no one called for help because I believe they were afraid of being arrested for taking drugs as well.”

Rose Humphries says the Anybody’s Child campaign helped her ‘break the wall of silence’. Photograph: Sarah Lee/The Guardian

The Humphries, from Bromsgrove in Worcestershire, said their sons were “good people whom we brought up well”.

As well as marching to demand MPs consider a radical new approach to the drugs issue, Rose Humphries said the Anybody’s Child campaign had helped her “break the wall of silence”.

“For years I could not speak about the boys to neighbours or friends. It was just something I could not talk about because of the shame and stigma. I erected this wall of silence around it because I thought people would judge me.”

Her husband defended his support for groups such as Transform, which lobbies for legalisation. “To those that say to us ‘how can you of all people be in favour of that?’, I respond simply by reminding them that prohibition has simply not worked and never will.”

Jane Slater, the campaign manager for Anyone’s Child, said she believed the debate on drugs legalisation was shifting. “Privately MPs we’ve met before have told us they think the current prohibition regime is wrong but then they say they are terrified of what the Daily Mail might say about them if they said they were in favour of liberalisation.”

Vince Cable, the departing Liberal Democrat leader, met the marchers as they made their way through his Twickenham constituency on Sunday.

While stressing he was not in favour of full legalisation, Cable said he supported decriminalisation for the possession of drugs. “I do follow the science which states that it would be much better to have the legalisation of cannabis, and decriminalisation – coupled with very tight regulation – of drug use generally,” he said.

But Brian Reed, who first lost his god-daughter and later his daughter Lydia to heroin laced with the powerful painkiller Fentanyl, said decriminalisation was not enough.

The 69-year-old retired industrial chemist and former chemistry teacher said: “Decriminalisation means that people will still die from contaminated drugs. Decriminalisation means that the recreational drugs trade is still in the hands of criminals who have no proper pharmaceutical knowledge and don’t care who dies. The trade should instead be put into the hands of professional people and the whole business regulated.”

As well as taking part in the march, which ends on the eve of the UN’s International Day against Drug Abuse and Illicit Trafficking, Reed intends to use his chemistry background to possibly save lives this summer.

“I’ve signed up again with The Loop [a charity that carries out drug safety tests at major events] to help at two major music festivals over the next couple of months. While we wait for the law to change it’s a small but practical way to save lives,” he said.