Home Secretary David Blunkett last week announced that cannabis will be downgraded from a class B to a class C drug.
According to an NOP survey for PRWeek, a third of all adults and teenagers over the age of 15 still do not understand what that reclassification means.
One in three of the UK population amounts to more than 12 million people.
The survey found widespread support for a communications offensive by the Government to make clear what the changes mean.
Eighty-seven per cent of those surveyed agreed or agreed strongly that more information needed to be made available on the legal implications of the new rules.
In parliament last week, Blunkett stressed the decision to reclassify was so that police resources could be better used to tackle class A drugs such as heroin and crack cocaine.
Legislation is set to be introduced by July 2003 so that police retain the power of arrest, but will only use it if there are aggravating factors such as public disorder issues, flagrant disregard for the law or a need to protect children.
The Government has already pledged to launch an education campaign on all drug abuse matters. The campaign will target the young and emphasise the dangers of class A drugs.