Concerns about the influx of so-called legal highs into the county has led to a warning by police about the serious harm they can cause.

The Government’s drugs tsar, Professor Les Iversen, chairman of the Advisory Council on the Misuse of Drugs (ACMD), has warned that the UK is being swamped and said he and his colleagues cannot keep tabs on all the substances available as there are now about 200 such chemicals flooding the market.

Mr Iversen told a public hearing last week that dealers are dodging laws by dressing the drugs up as plant food and warned that overdose risks were “clearly immense”.

Last year, two legal highs – mexxy and black mamba – were made illegal class B drugs.

West Yorkshire Police drugs co-ordinator Bryan Dent said the national publicity reiterated what West Yorkshire Police and its partners had been saying for some time.

“We are concerned about the number of new substances which appear in the county,” he said. “We are always keen to give out the message that these substances, although many legal, are not safe to consume.

“The police and our front- line colleagues in other services are regularly called to people who have got into difficulties as a result of taking these substances, which are designed to mimic illegal substances.

“As a result, it has resource implications for police, ambulance, A&E and drug treatment services, not to mention the danger caused to the person who has taken the substance and the distress it causes for family and loved ones.

“Again I would take this opportunity to warn anyone who is tempted to take such substances, while they may be colourfully and professionally packaged, you will have no idea what the contents are and the fact that you can purchase them legally does not mean that they are safe. I would hate for any person in West Yorkshire to consume legal substances and seriously damage their health.”

Prof Iversen said: “Our problem is to know how many of these are really being used in this country and how harmful are they.

“This is difficult because we can’t possibly address all classes of compounds at once, unless we and the Government can think of cleverer ways of regulating.”

Mr Iversen said the minute doses were often supplied as a diluted solution dropped on a piece of blotting paper.

He said: “However, we know it’s also possible to buy powders of some of these new compounds, sprays and fluids. So this is an area we’re looking at with a great deal of caution and worry.”