The Psychoactive Substances Act, which comes into force in the UK next month, makes it illegal to supply drugs which have a direct effect on mental processes, unless the drug is exempted.
Ministers had indicated that poppers would be included in the ban.
But advisers say poppers have only a "peripheral" effect on the brain.
As a result, the Advisory Council on the Misuse of Drugs said poppers - the name given to a group of chemicals called alkyl nitrites - "do not fall within the scope of the current definition of a psychoactive substance".
The council's guidance is contained in a letter which also sets out the harms linked to the use of poppers.
It says there were 11 deaths related to poppers in England and Wales from 1993 to 2013.
In January, Conservative MP Crispin Blunt said he used poppers and said banning supply would be "fantastically stupid".
Government drugs advisers have said "poppers" - used by many gay men to enhance sexual experience - will not be covered by a ban on "legal highs".