Cannabis users will not face fines from today, despite it being upgraded to a Class B drug.
Police will be unable to hand out the £80 fines intended as part of a new "three strikes" regime.
The Home Secretary Jacqui Smith decided to move the drug from Class C to Class B last year, because of fears over the impact of stronger strains of "skunk" on the mental health of young people.
Under the new regime, police should issue a warning to anyone caught with cannabis for a first offence, and give second time offenders an £80 fine and penalty notice.
Anyone given a third "strike" will be arrested and could face an unlimited fine and a prison sentence of up to five years.
The order in Parliament making cannabis use an offence punishable with a Penalty Notice for Disorder (PND) was scheduled to pass last week.
It was bundled up with a group of other offences, including mini-cab drivers hawking for business.
But because of opposition to some of the changes, the package of measures was withdrawn for consultation, the Ministry of Justice said.
The cannabis order is not due to be debated in the House of Lords until Monday and will come in to force on Wednesday morning.
It has also emerged that not all police forces will record cannabis warnings, meaning repeat offenders could escape fines or prosecution. A system for recording all cautions is not due to be introduced until next year.
Home Office Minister Alan Campbell warned the average age of first-time cannabis users is now 13.
He said: "Cannabis is a harmful drug and while fewer people are taking it than before, it poses a real risk to the health of those who do use it.
"I am extremely concerned about the use of stronger cannabis - skunk - and the harm it can cause to mental health.
"We are reclassifying cannabis to protect the public and future generations. That is why, together with reclassification, it is crucial that we are communicating with young people through the FRANK drug awareness campaign to warn them about the consequences."
Ms Smith, who has admitted smoking cannabis at university, announced the decision to reclassify the drug in May last year.
In Class B, cannabis users who are successfully prosecuted can face an unlimited fine and up to five years in prison.
It was declassified by David Blunkett in 2004, putting it in the same category as body building steroids and some tranquillisers.
A report out last week revealed record numbers of young people were being given help for drug and alcohol abuse because of an increase in the number of treatment places.
Marjorie Wallace, chief executive of the mental health charity SANE, said: "SANE has been campaigning for many years over the mounting evidence of direct links between cannabis and later schizophrenia, and we support the proposal to reclassify it as a Class B drug.
"Cannabis, especially in its more toxic varieties such as skunk, can double the chance of developing severe mental illness in a significant minority of people, particularly the young whose brains are still developing.
"While we do not yet know the cause of psychotic illness, or the ways in which drugs such as cannabis may trigger breakdown, relapse, and worsen outcomes, we need to maintain a clear message that it is dangerous to the 10-20% of people who may be at risk but do not know it."
Former Metropolitan Police deputy assistant commissioner and London mayoral candidate Brian Paddick told BBC Breakfast that changing the classification of the drug would make little difference to the numbers of people using it.
"I don't know that there are any young people who are making the decision to smoke cannabis saying 'Hang on - what class is this drug in?"'
Mr Paddick pioneered a "softly softly" approach towards cannabis use as a commander in Lambeth, south London.
He said: "If the police have a problem with stronger drugs like heroin and cocaine then they will concentrate on those, as we did in Lambeth.
"The Magistrates Association have said this morning that the Government have moved it to a Class C plus or a B minus.
"The three strikes approach only applies to cannabis and not to any other sort of drug.
"It is a very confusing message."