The mysterious programme is being co-ordinated by Russia’s Federal Service for Military-Technical Co-operation, according to state-controlled Russian news agency RIA Novosti. Both countries have continued to operate conventional attack submarines alongside their nuclear fleets.
Defence experts say they are cheaper and have some advantages over their nuclear sister ships, especially in inshore deployments.
They are also exportable and there have been suggestions the new submarine is not intended for the domestic navies of either country but is instead being developed as a commercial consideration.
Russia and China both compete on the international market, mainly with conventional attack submarines and Beijing enjoys an ever-increasing share of this market with sales to Thailand, Bangladesh and Pakistan.
The two superpowers have demonstrated growing levels of defence co-operation in recent years and only this week elite troops both sides joined forces to compete in this year’s International Army Games.
Their apparent closeness has sparked warnings Moscow and Beijing are preparing their forces for a day when they might fight alongside one another, in a dire threat to its enemies.
The games are just one aspect of increased Chinese-Russian joint military training in recent years.
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They have conducted multiple joint naval drills every year since 2012, including flashpoints such as the South China Sea and the Baltic Sea.
China and Russia also held their first joint naval exercise with Iran in the Gulf towards the end of last year.
That followed their first joint strategic bomber patrol in July 2019 over the waters between the Korean Peninsula and Japan, which sparked protests from both Seoul and Tokyo.
Russia has a proud tradition of submarine building, creating many of the most powerful and largest subs in the world, and helped China develop its own submarine building industry during the Cold War.
More recently Russia supplied China with Kilo Class diesel-electric attack submarines.
But China has gone its own way with submarine design and may now be ahead of Russia in propulsion technology.
China is building Air Independent Power (AIP) submarines while Russia has struggled in this area.
Analysts said combining the hull technology of one nation with the combat systems and weapons of another opened up a wide range of possibilities such as fitting a Chinese vessel Russian sonar and weapons or fitting a Russian submarine with Chinese battery and AIP technology.