Jens Stoltenberg cited the WannaCry ransomware attack as an example of a “serious cyber attack” that would be viewed by Nato as requiring a response from all members of the alliance.

The attack in May 2017, which exploited a weakness in some Microsoft Windows operating systems, forced the NHS to cancel non-emergency operations.

It is estimated to have affected 70,000 devices including computers and MRI scanners.

The attack, in which the perpetrators demanded payments to fix infected computers, spread to dozens of countries and cost hundreds of millions of pounds to put right.

The US and UK have publicly accused North Korea of being behind the attack. But other state actors including Russia, China and Iran have been blamed for other cyber attacks in the West.

Mr Stoltenberg said that a similar attack could in the future trigger Article 5 of the Nato founding treaty, which commits  member nations to treating an attack on one member as an attack against all.

The collective defence commitment has been invoked only once in Nato history - by the US after the al-Qaida terrorist attacks on New York and Washington on 11 September 2001.

In an article for Prospect magazine, Mr Stoltenberg  wrote: “We have designated cyberspace a domain in which Nato will operate and defend itself as effectively as it does in the air, on land, and at sea.

“This means we will deter and defend against any aggression towards allies, whether it takes place in the physical world or the virtual one.”

Mr Stoltenberg added: “The 2017 WannaCry virus crippled computers in hospitals across the country, cancelling thousands of scheduled operations and costing the National Health Service millions of pounds.

“Even Nato is not immune to cyberattacks and we register suspicious activity against our systems every day.”

The Nato chief said the alliance is establishing a new Cyberspace Operations Centre in Belgium, and could also now “also draw from allies’ national cyber capabilities for NATO missions and operations”.

He also praised action by the Netherlands, with the help of British intelligence agencies, in foiling an attack by Russia on the Organisation for the Prohibition for Chemical Weapons (OPCW) in The Hague.

At the time, the OPCW was investigating the attempted assassination of Sergei Skripal and his daughter Yulia in Salisbury in a chemical weapons attack using a military-grade nerve agent.

Mr Stoltenberg said collaboration was vital, explaining that “no single country alone can secure cyberspace”, adding: “But by cooperating closely, sharing expertise, we will not only survive, but thrive in the new digital age.”

He went on: “Cyberspace is the new battleground and making Nato cyber ready, well-resourced, well-trained, and well-equipped-is a top priority as we look towards the Nato summit in London in December and beyond.”