Supporters describe the treaty as a historic achievement but the nuclear-armed states have dismissed the ban as unrealistic, arguing it will have no impact on reducing the global stockpile of 15,000 nuclear weapons.
Led by Austria, Brazil, Mexico, South Africa and New Zealand, 141 countries have taken part in three weeks of negotiations on the treaty that provides for a total ban on developing, stockpiling or threatening to use nuclear weapons.
Advocates hope it will increase pressure on nuclear states to take disarmament more seriously.
"This will be a historic moment," Costa Rica's ambassador, Elayne Whyte Gomez, the president of the UN conference on the treaty, said on the eve of the adoption.
"The world has been waiting for this legal norm for 70 years," she said, calling it a "response for humanity."
None of the nine countries that possess nuclear weapons -- the United States, Russia, Britain, China, France, India, Pakistan, North Korea and Israel -- took part in the negotiations.
Even Japan -- the only country to have suffered atomic attacks, in 1945 -- boycotted the talks as did most NATO countries.
US Ambassador Nikki Haley came out strongly against the ban when negotiations opened on March 27, saying "there is nothing I want more for my family than a world with no nuclear weapons, but we have to be realistic."
"Is there anyone that believes that North Korea would agree to a ban on nuclear weapons?" she asked.
- No more prestige -
Nuclear powers argue their arsenals serve as a deterrent against a nuclear attack and say they remain committed to the nuclear Non-Proliferation Treaty (NPT).
Impatience however is growing among many non-nuclear states over the slow pace of disarmament as are worries that the weapons of mass destruction will fall into the wrong hands.
Disarmament campaigners say the treaty will go a long way in increasing the stigma associated with nuclear weapons and will have an impact on public opinion.
"The key thing is that it changes the legal landscape," said Richard Moyes, director of the British-based organisation Article 36.
"It stops states with nuclear weapons from being able to hide behind the idea that they are not illegal."
"This is really about removing the prestige from nuclear weapons," said Beatrice Fihn, director of the International Campaign to Abolish Nuclear Weapons.
"They are seen as something very valuable and as giving power. This is supposed to remove that."
During a meeting at the General Assembly, the treaty is expected to be adopted by consensus by the conference of nations that has negotiated the document without the nuclear powers and their allies.
After its adoption, the treaty will be open for signatures as of September 20 and will enter into force when 50 countries have ratified it.
During a vote at the UN General Assembly in December, 113 countries voted in favor of starting negotiations on the new treaty while 35 opposed the move and 13 abstained.
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