All five legally recognised nuclear weapon states – China, France, Russia, Britain and the US – are either deploying new nuclear weapon delivery systems or have announced programmes to do so, according to an authoritative study.

India and Pakistan are also developing new systems capable of delivering nuclear weapons and are expanding their capacities to produce fissile material for military purposes. And there is an emerging consensus in the expert community that North Korea has produced a small number of nuclear weapons, as distinct from rudimentary nuclear explosive devices.

These are the conclusions of the latest annual survey by the Stockholm International Peace Research Institute (Sipri), which says nine states – the US, Russia, the UK, France, China, India, Pakistan, Israel and North Korea – possess a total of approximately 4,000 operational nuclear weapons.

A decrease in the overall number of nuclear weapons is due mainly to Russia and the US, which together account for more than 93% of all nuclear weapons, further reducing their inventories under the terms of the New Start treaty, the report says.

"Once again this year, the nuclear weapon-possessing states took little action to indicate a genuine willingness to work toward complete dismantlement of their nuclear arsenals. The long-term modernisation programmes under way in these states suggest their views that nuclear weapons will remain deeply embedded elements of their strategic calculus," said the Sipri researchers, Shannon Kile and Phillip Patton Schell.

The US plans to spend up to $350bn (about £200bn) over the next decade on modernising and maintaining its nuclear forces, including designing a replacement for its existing Trident submarines beginning in 2031, the study says. This would have implications for Britain’s future nuclear weapons programme.

Russia is building a new class of nuclear missile submarines and replacing all its Soviet-era intercontinental ballistic missiles with mobile, multiple-warhead versions of the existing SS27 system.

China is expanding its conventional ballistic missile programme and has deployed dual-capable medium and short-range ballistic missiles. Mixing conventional and nuclear missiles poses a critical risk of mistaken escalation of a conflict, as an adversary would not be able to determine whether the missile fired is armed with a conventional or nuclear warhead, Sipri warns.