The 10 Dutch cities that are taking part in the ongoing regulated marijuana experiment will only have legally-produced types of the soft drug on sale from April next year.

Justice minister David van Weel said on Tuesday that the green light had been given for the experiment to continue and that means 10 towns and cities will say goodbye to illegal supplies.

The experiment was launched in Breda and Tilburg at the end of last year and in June was extended to Groningen, Zaanstad, Almere, Arnhem, Nijmegen, Voorne aan Zee, Heerlen and Maastricht.

Up to now, cannabis cafes have been offering both regulated and “illegal” marijuana. The aim of that stage in the programme was to “optimise” the “quantity, quality and diversity” of the marijuana on offer.

From April 7 the 75 cannabis cafes, or coffeeshops, will limit to supplies from regulated growers only. Currently four growers are offering licenced marijuana but that is being extended to seven.

The experiment aims to phase out the Dutch gedoog (tolerated) policy that has decriminalised the possession of small amounts of cannabis and licenced sales outlets, but makes it illegal to produce and sell in bulk.

This “front door, back door” system was introduced to draw a line between so-called “soft” and “hard” drugs in the 1970s when Amsterdam became world-renowned as a drug haven.

But this civic compromise has left the door open to continued criminal rule, with robberies, violence and money laundering still plaguing much of the industry. Breda’s mayor Paul Delpa, one of the main campaigners for a regulated system, told Dutch News last year the experiment is fundamentally about safety.

“The Dutch policy for weed is quite devious. People can buy it legally in coffeeshops, but the production of the weed and the buying part (the back door) of the shop owners is illegal. That means there’s a big criminal world that thrives on producing the weed and selling it to the shops. That needs to change.”

Read the justice ministry report, in English

The new coalition had been concerned about the experiment but a majority of MPs voted against the far-right PVV’s plan to call a halt. They also rejected a bid to allow Amsterdam’s eastern district to join in at the last minute.

The new phase in the experiment is scheduled to run for four years.