Nato's secretary general Jens Stoltenberg has revealed a "transformative" summit for the alliance this week will decide to boost the troop numbers in its high-readiness response force to 300,000 from 40,000 currently.

The plan being discussed when the meeting gets underway in Madrid on Tuesday "constitutes the biggest overhaul of our collective deterrence and defence since the Cold War".

The alliance would decide on a new "strategic concept for a new security reality" when leaders will come together for a meeting at least as important as the 1997 gathering in the Spanish capital that began the alliance's post-Cold War expansion, when Poland, Hungary and the Czech Republic were invited to join.

The summit represents a watershed moment, amid discussions on upgrading the threat designation of Russia, significantly strengthening Nato's posture in eastern Europe and adopting a wholly new strategic concept.

The Russian invasion of Ukraine has fundamentally changed the organisation’s posture, leading to a major shake-up of defence budgets and armed forces.

Over three days in the Spanish capital, leaders from the 30 Nato nations will hammer out a range of issues such as supplying more weapons to Ukraine, China, climate change and the entry of new member states.