China is undertaking a dramatic overhaul of its nuclear weapons in an effort to modernise and expand its arsenal.

One of the world's leading arms control experts has said that the Chinese have realised that their nuclear weaponry has fallen behind those of other major powers and might not survive a first strike.

Bates Gill, head of the Stockholm International Peace Research Institute (Sipri), said that as a result it was developing more flexible delivery systems, including from submarines, as well as the capacity to use multiple warheads.

"Among the major nuclear powers China stands out in its effort to modernise, expand and improve its nuclear weapons capability," he said at a conference in Beijing.

China's first nuclear test took place amid huge patriotic pride in 1964.

But Chairman Mao was famously ambiguous about such weapons, once calling them "paper tigers".

Its arsenal, estimated at between 100 and 200 warheads, is the smallest of the big powers – the United States, Russia, Britain and France. The US is currently updating its missiles and warheads.

China now has a stated policy of never using nuclear weapons against a non-nuclear country and never as a "first strike". But Dr Gill said its static nuclear delivery system had left it vulnerable to a first strike.

A sea-based capability would "make it less likely that an adversary could wipe out the possibility of a response," he said.