The risk of a nuclear war breaking out because of a 'catastrophic error' is at its highest ever point, the UN has warned in an ominous report.

Complex automated systems could malfunction and start a chain of events which could claim millions of lives, according to a report by the UN's Institute for Disarmament Research (UNIDR).

The document concludes: 'Nuclear deterrence works—up until the time it will prove not to work. The risk is inherent and, when luck runs out, the results will be catastrophic.'

The document concludes: 'Nuclear deterrence works—up until the time it will prove not to work'

It warns that interconnected sensors, drones and networks mean that one error could very quickly escalate, and warns that the risk of 'accident, misunderstanding or inadvertent escalation' are 'too high'.

Its report states: 'The complex interactions and tightly coupled systems linked to nuclear arsenals (like those for early warning, and launch command and control) have made "accidental war" more likely.'

And it continued: 'A greater reliance on automated systems can lead to misplaced confidence while introducing new points of vulnerability.'

The document stated that more smaller states are becoming nuclear powers, and voiced fears these would have 'less secure control' over their weapons - making them vulnerable to terrorists.

Automated systems leads to 'misplaced confidence' and also leads to 'new points of vulnerability', UNIDR warns

Referring to a 2013 UN General Assembly meeting, the report said: 'As long as nuclear weapons existed, there remained the risk of their use, either intentionally or accidentally, or of their further proliferation.

'The high-level meeting reaffirmed that the only guarantee against the threat of nuclear weapons was their total elimination.'

It cites Cold War examples in which the world appeared on the brink of nuclear war, including a 1983 flashpoint following a malfunction in Soviet weapons systems.

In that instance Stanislav Petrov, a lieutenant colonel in the Soviet Air Defence Force, correctly ruled that five missiles had not been launched by the US, in spite of what his equipment was telling him.

His scepticism, UNIDR experts stated, prevented the outbreak of nuclear war.

The risk is amplified still further by the possibility that states such as the USA are examining the possibility of nuclear-enabled drones, the report said.

Russian arms expert Pavel Podvig wrote: 'The spread of other systems, such as cruise missiles and drones, and their increasingly frequent use in military conflicts can also add to the complexity of the situation, as can the development of capabilities to detect missiles.'