The legalisation of cannabis should be tested in London to improve public health and stop young people being drawn into crime, a London mayoral candidate has said.

Siobhan Benita, the Lib Dem candidate for next year’s election, said the idea of legalising the drug was “no longer controversial” and the serious crime in the capital meant it was the right place for the idea to be trialled.

“Illegal drugs activity, especially in the capital, is a big part of pulling young people into serious violence,” she told the Observer. “I want to remove power from those gangs. My question would be, why haven’t we done this yet? It’s not controversial any more. We’ve got enough examples now of countries around the world and we can compare and contrast how they have done it. We now have lots more evidence on where it is working well.”

She said legalisation, which would free up police time, had been supported by prominent former police officers. “This has been a Lib Dem manifesto commitment for several years, but what brought it to the fore for me was my work with the cross-party commission on serious youth violence. There was clear evidence coming out of it that the more exposure [young people] had to the illicit drugs market, the more likely they were to be exposed to serious violence or know people who were.

“With resources stretched as well, police don’t want to be diverted into activity on cannabis. If you can regulate and make sure the quality is much safer, it removes the need for police to be looking out for that.”

Legalisation has growing support among MPs. Several Tories now privately say they believe the idea is gaining ground. Benita cited comments from former Metropolitan police chief Bernard Hogan-Howe last year, in which he called for an “urgent review” of Britain’s cannabis laws. He said the US had shown how changes could be made in a safe way.

“I’ve not seen clear evidence to say change the law now,” he said. “But I have seen clear evidence to say let’s review it, in a time-limited way, not a kicking-into-the-long-grass way. We need to get on with it. We’re lucky – we’re not the pioneers and we can learn from others’ mistakes. The evidence is out there and it shouldn’t be ignored.”

Benita also called for a “youth happy hour” to tackle youth crime and violence. It would see venues across London lay on activities for them between 4pm and 6pm.

“We know there is a problematic time when young people are particularly vulnerable to getting involved in criminal activity or serious violence – that’s as they leave school,” she said.

“I don’t want to be a mayor that says, ‘I’d do this if I had the resources’. The message I’m getting is that there are organisations that have the venues and volunteers. There are a lot of public buildings across London that lie empty.”