Police raided a dozen shops in Wakefield, Bradford and Leeds and seized 155 packets of legal highs.
But the substances have now been reclassified as Class A drugs.
Government changes to the Misuse of Drugs Act 1971 saw legal highs that mimic the effects of drugs such as cocaine, ecstasy, amphetamine and cannabis become Class A drugs.
No one was arrested but staff at the stores were advised of the change in the law.
The raids took place earlier this month as part of Operation Nightshot.
It was launched in Leeds in December 2012 and has seen a number of landmark prosecutions against high street ‘head shops selling so called legal highs.
Insp Ian Williams, Wakefield Police’s neighbourhood inspector, has called on shops in the city to stop selling legal highs.
He said: “The government is making certain brands illegal all the time and if the shops are not keeping up with the changes, they could be selling something that is illegal.
“They are being used in the bars and clubs, and even in schools.
“We need to clamp down now or it will become a bigger problem in the future.”
Det Con Jamie Hudson said there is a misconception that products being sold as legal highs are safe when they can be extremely dangerous.
He added: “Anyone who trades in psychoactive substances should be aware that it is their responsibility to keep up to date with any changes in legislation.
“Ignorance is not a defence and we will take action where it is suspected that criminal offences are being committed.”
West Yorkshire Police drugs co-ordinator Bryan Dent has warned that people who use legal highs are playing Russian roulette with their lives.
He said: “People are buying these substances but have no idea what is in them.
“They don’t know the short or long term effects and what consequences it will have on their health.”