Two lethal "legal highs" - including a substance linked to 37 UK deaths - have been banned as class A drugs.
Stimulant Serotoni - known technically as 4,4'-DMAR - and another substance not currently found in the UK called MT-45 have both been controlled under the Misuse of Drugs Act 1971, crime prevention minister Lynne Featherstone said.
Serotoni, which has been linked to 37 deaths in the UK, mostly in Northern Ireland, is usually bought in tablet or powder form, while deaths have been connected to MT-45 in Europe and the US.
Possession of a Class A drug can be punished by up to seven years in prison and an unlimited fine, while supply of a Class A drug can lead to life in prison and an unlimited fine.
The move comes after the Advisory Council on Misuse of Drugs (ACMD) recommended that both substances should be banned.
Both MT-45 and Serotoni have recently been subjected to risk assessments by the European Monitoring Centre for Drugs and Drug Addiction.
Professor Les Iversen, chair of the ACMD, previously said: "Both of these substances pose serious dangers and we are advising they should be banned in the UK."
Last month, a fresh row over drug use erupted when an official Government study found treating drug possession as a health problem rather than a criminal matter has no impact on levels of substance misuse.
The Home Office report said drug use is not affected by the ''toughness'' of a country's enforcement on possession of substances.
Liberal Democrat MP Norman Baker resigned as crime prevention minister in the wake of the report's publication, claiming that working with Home Secretary Theresa May had been like ''walking through mud'' because she treated the Home Office as a Conservative department despite being in coalition.
Mr Baker argued that the current approach to drugs was no longer tenable and claimed the bombshell report had been suppressed by the Conservative Party.