Campaigners have hailed a "new chapter" after a key step by the United Nations towards banning nuclear arms.
Honduras has become the 50th country to ratify the Treaty on the Prohibition of Nuclear Weapons so it will now come into force in 90 days' time.
New Zealand and many Pacific nations including Samoa, Fiji, Niue, Tuvalu, Cook Islands and Kiribati are among the signatories.
But what it will actually achieve remains in doubt because the five recognised nuclear powers have not signed the accord.
Supporters hope it will nevertheless have a deterrent effect.
What's in the treaty?
The accord was approved by 122 countries at the UN General Assembly in 2017 but needed to be ratified by at least 50 before being enacted.
It declares that those countries that ratify it must "never under any circumstances develop, test, produce, manufacture, otherwise acquire, possess or stockpile nuclear weapons or other nuclear explosive devices".
The treaty outlaws the use or threat to use nuclear arms, and bars signatories from allowing "any stationing, installation or deployment of any nuclear weapons or other nuclear explosive devices" on their territory.
What has the reaction been?
The International Campaign to Abolish Nuclear Weapons (Ican) described the 50th ratification as heralding "a new chapter for nuclear disarmament".
Beatrice Fihn, the head of Ican, which was awarded the Nobel Peace Prize in 2017, said: "Decades of activism have achieved what many said was impossible: nuclear weapons are banned."
The president of the International Committee of the Red Cross, Peter Maurer, said: "Today is a victory for humanity, and a promise of a safer future."
A statement from the UN Secretary-General, Antonio Guterres, described the move as "a meaningful commitment towards the total elimination of nuclear weapons, which is the highest disarmament priority of the United Nations".
There has been no immediate reaction from the five main nuclear powers - the US, Russia, China, the UK and France. But the US and the UK made clear their opposition in 2017.
The UK said at the time that, while committed to a nuclear-free world, the government does not believe the treaty will bring about an end to nuclear weapons and could undermine existing efforts to do so, such as the Nuclear Non-Proliferation Treaty.
The US, in a letter seen by Associated Press, wrote to the treaty signatories saying the accord "turns back the clock on verification and disarmament".
Stocks of weapons
It is believed there are about about 14,000 nuclear weapons worldwide, but considerably fewer than the 70,000 known to exist in the mid-1980s.
The US and Russia have the most, followed by France, China and the UK. India, Pakistan and North Korea are also nuclear powers. Israel is widely believed to have nuclear weapons, but refuses to confirm or deny.
What is being done to eliminate them?
The Non-Proliferation Treaty, backed by 190 countries in 1970, commits countries which signed up, including the US, Russia, France the UK and China, to reducing their stockpiles and bars others from acquiring nuclear weapons.
India, Pakistan and Israel did not sign up and North Korea left in 2003. The US, Russia and the UK have been reducing their inventories.
Russia and the US are trying to extend their last remaining nuclear arms agreement which is due to expire in February.
New Start, signed in 2010, limits the number of long-range nuclear warheads each can possess to 1550.
The US recently pulled out of another treaty, the Intermediate-Range Nuclear Forces treaty signed during the Cold War, after accusing Russia of violating it.