A separate proposal to downgrade Tanzania's elephants to a lower level of protection, which allows commerce if it is monitored and deemed sustainable, was also rejected.
The ivory trade was banned in 1989, but two one-off sales were allowed in 1999 and 2008.
Tanzania and Zambia had proposed selling their stockpiles to Japan and China, the only countries which have asked to purchase it.
The sales would have netted millions of dollars for both countries.
"We are sitting on a treasure that we are not allowed to use to help our population, to help the poor build schools and roads," said Stanslaus Komba, from Tanzania's ministry of natural resources.
Zambia said it would have used the funds raised for conservation projects.
The bids come amid renewed poaching of the elephant and a dramatic surge in seizures of illegal ivory in 2009.
About 25 tonnes of ivory culled from an estimated 2,600 elephants were confiscated last year, mainly in Asia, according to Traffic, a wildlife trade monitoring group.
The recent surge in seizures by law enforcement officers has caused experts to question whether one-off sales stimulate illegal trade rather than stem it, as was once thought.
Speaking to Al Jazeera, Will Travers, chief executive of the Born Free wildlife advocacy group, explained why he objected to the proposals.
Travers said: "We have the high level of poaching, on top of that we have the increase of smuggled ivory that has been intercepted in the world, and the high price of raw ivory which is fetching as much as $1,800 a kilo in parts of the Far East.
"Our fear was [that] more ivory ... being put in the market would stimulate demand, not satisfy demand, and that the situation in Africa would get even worse."
The last permitted sale of ivory in 2008 saw Botswana, Namibia, South Africa and Zimbabwe releasing stocks.