Drug campaigners have warned that a batch of cannabis adulterated with tiny glass beads which they say could pose a risk to health has flooded the UK market. Anecdotal reports suggest it is being sold in almost every part of the country.

The charity Drugscope is issuing a warning. "We want to make people aware of it from a public health point of view. If you are smoking this stuff and taking it into your lungs it's not good news."

The fact that "grit weed", as it is being called, is so widespread suggests that contamination is happening at an early stage in the production process. "It seems to be being done on an industrial scale," said Harry Shapiro of Drugscope.

The dealers' motivation seems to be to bump up the weight of their product. They appear to be doing this by spraying plants with the reflective element from the paint used on road lines. The tiny reflective glass beads become imbedded in the leaves.

"It looks perfectly normal. In fact it looks good quality," said Derek Williams of UK Cannabis Internet Activists, which campaigns against cannabis prohibition.

Contributors to its internet forum have said the contaminated leaves can be identified by rubbing them between wetted fingers. The residue will feel gritty if chewed.

The first reports of grit weed circulated in late summer, but the groups have only recently received pictures of the beads taken with an electron microscope by an anonymous scientist. There have been rumours of users experiencing a tight chest for days after smoking grit weed, but a spokesperson for the British Thoracic Society said that the contamination was unlikely to be dangerous because the particles are too big to be inhaled into the lungs.

An analysis carried out by the French Observatory of Drugs and Drug Addiction found that the glass particles are between 0.02 and 0.3 millimetres across. But Richard Russell, a consultant at Imperial College London, said few particles above 0.015 millimetres would be able to pass into the lungs because they are too big.

"It is likely that they will deposit in the mouth or the throat," said Dr Russell. Here they would most likely pass harmlessly through the system. Using a filter would stop them entering the mouth.

Dr Russell said that smokers should be aware that cannabis smoke causes emphysema, lung inflammation and cancer. "You are likely to do more damage from the marijuana than these particles."

Mr Williams said that the widespread contamination highlighted the problems which stem from cannabis prohibition. "Cannabis is called a controlled drug, but there is no control over the supply side. It's a completely underground product," he said.