The New Start treaty between the US and Russia will expire on Thursday, removing the last remaining mutual limits on the world’s two biggest nuclear arsenals.

The milestone will be a death knell for more than five decades of arms control at a time of surging global instability, contributing to a general collapse of the rules-based international order established after the second world war.

Alexandra Bell, the president and chief executive of the Bulletin of the Atomic Scientists, said: “When it comes to nuclear risks, everything is trending in the wrong direction over the course of 2025. Nuclear risks have become more complex, more dangerous and we have seen leaders fail in their obligation to manage those risks.

“And we are two days away from watching the United States and Russia fritter away half a century of work to maintain nuclear stability between the two largest nuclear states.”

Dmitry Medvedev, who signed the New Start deal with Barack Obama in 2010 when Medvedev was Russian president, said the treaty’s expiry should “alarm everyone”.

“When there is an agreement, it means there is trust but when there is no agreement, it means that trust has been exhausted,” Medvedev, who has become an outspoken Moscow hawk, said.

Arms control advocates have appealed to the world’s two nuclear weapon superpowers to act at the 11th-hour to salvage the treaty, which limits each country’s deployed strategic arsenal to 1,550 warheads and the total number of delivery systems (missiles or bombers) to 800.

In September, Vladimir Putin proposed extending the New Start agreement by another year, which Donald Trump at the time said “sounds like a good idea”.

But the remarks did not appear to have been followed by any substantive negotiations. Moscow said it had not received a formal response to Putin’s one-year suggestion.

“If it expires, it expires,” Trump told the New York Times in January. “We’ll just do a better agreement.”

A White House official later briefed that Trump wanted a deal that also involved China, which has a considerably smaller arsenal of 600 warheads, very few of which are deployed and ready for use, according to Federation of American Scientists (FAS) estimates.

By comparison the FAS assesses the US arsenal at 5,177 (including stockpiled and retired warheads) and Russia’s at 5,459.

Jennifer Kavanagh, the director of military analysis at the Defense Priorities arms control advocacy group, described Trump’s hopes of a “better deal” after New Start’s expiry as “wishful thinking”.

“If the administration thinks getting a new ‘better’ treaty after this one lapses will be easy, they are mistaken,” Kavanagh said. “An agreement with Russia is … likely a requirement to get China onboard with nuclear arms control. Trump may be the ultimate dealmaker, but in this case he would be better off hanging on to the agreement he has a little longer before trying to get a better one.”

Unofficial reports suggested the Trump administration could make an announcement about its nonproliferation ambitions, but only after New Start has expired.

As well as imposing numerical limits on arsenals, New Start established an extensive system of mutual monitoring, data sharing and inspections, though Putin suspended Russian participation in that aspect of the treaty in 2023 in response to US support for Ukraine in the face of his full-scale invasion.

The end of New Start would be a last gasp for worldwide arms control, which has been unravelling over a number of years. Agreements limiting missile defence systems, intermediate-range forces and mutual overflight rights have already crumbled. The nuclear weapons powers have invested hundreds of billion dollars in modernising their arsenals, Putin and Trump openly brandish their countries’ nuclear arsenals in their rhetoric, and the US president has also threatened to end his country’s moratorium on nuclear tests.

Daryl Kimball, the head of the Arms Control Association in Washington, said the end of New Start could ignite a new arms race quite rapidly.

“There are many in the nuclear weapons establishment who want to rapidly build up the size of the US force in order to counter China’s strategic buildup,” Kimball said.

The demise of New Start could in turn threaten the 1970 nuclear nonproliferation treaty (NPT), under which states without nuclear weapons pledged not to acquire them, as long as the weapons states made good faith efforts to disarm. The NPT is next up for review this year.

“This would represent a direct violation of the United States’ legal obligations under the NPT, and would shake the foundations of yet another core set of rules undergirding the important, if imperfect, rules-based order,” Kimball said.

One of the historical justifications for nuclear deterrence has been that it made the world more stable, by making nuclear powers cautious about risking direct conflict. But even before the death of New Start, there were many signs that rival nuclear arsenals were losing any stabilising effect they might have had, particularly after the Russian invasion of Ukraine in 2022, consequent western arming of Ukraine and a steep rise in friction along the Russia-Nato border.

“Put simply, nuclear weapons are no longer functioning as a decisive factor in global security,” Alex Kolbin, a nuclear weapons analyst, said in the Bulletin of the Atomic Scientists, in a piece last week titled Nuclear Deterrence Is Dying. And Hardly Anyone Notices.