Two "legal highs" will be temporarily banned for 12 months as the government assesses their long-term psychological and physiological effects and considers a permanent ban.
Benzo Fury and NBOMe will be illegal to buy, make, supply or import for the next year while the Advisory Council on the Misuse of Drugs (ACMD) decides whether they should be permanently controlled.
The Home Office announced the move following reports more than 200 legal highs were being monitored by European drugs experts.
The Home Office's temporary class drug order means it is not illegal to possess either drug but police and customs have the power to seize and confiscate them.
The stimulant Benzo Furty was previously available to buy online. it was marketed as "research chemical pellets" not for human consumption.
It acts as a stimulant with similar effects to amphetamines such as speed and like ecstasy. The drug is also known to give the user a high dose of energy and experience colours and sounds more intensely.
NBOMe is a psychedelic that acts like a milder form of LSD.
Minister for crime prevention Jeremy Browne said: "Drugs ruin lives and cause misery to families and communities and the use of this power shows how the UK is leading the way in cracking down on new psychoactive substances.
"This temporary class drug order will protect the public and give our independent experts time to prepare advice, while enabling law enforcement partners to target traffickers.
"Making drugs illegal is only part of the solution, however. Users of these substances need to understand that although they are described as legal highs, they are by no means safe."
The drugs charity Angelus accused the government of not doing enough.
The founder of Angelus, Maryon Stewart, said: "These are dangerous substances: Benzo Fury is a strong stimulant and NBOMe is a powerful psychedelic. The government is right to be concerned about them but the answer cannot be just another series of bans. That doesn't change how young people think or act and to present it as such is just papering over the cracks.
"Drugs education is still optional in this country when young people need to learn about the massive risks they are taking with legal highs. We also want the government to take firm action against these reckless retailers, like they have in Ireland and Portugal."