Nato will make clear that Russia is the “most significant and direct threat” to security when leaders gather in Madrid this week, the organisation’s secretary general said.

Boris Johnson will join Nato allies in Spain on Tuesday night for talks which will set a new direction for the alliance in response to Russia’s invasion of Ukraine.

Nato’s secretary general Jens Stoltenberg said the summit would be “transformative” for the alliance, with the number of troops in the alliance’s high-readiness response force soaring from 40,000 to more than 300,000.

The plan being discussed at Madrid “constitutes the biggest overhaul of our collective deterrence and defence since the Cold War”,” he said.

The alliance would decide on a new “strategic concept for a new security reality”.

It would be “a fundamental shift in Nato’s deterrence and defence” and the leaders would pledge “support to Ukraine now, and for the future”.

“Our new concept will guide us in an era of strategic competition,” he told reporters in Brussels.

“I expect it will make clear that allies consider Russia as the most significant and direct threat to our security.”

But the plan will also address China and the “challenges that Beijing poses to our security, interests, and values”.

The dramatic expansion of high-readiness forces will “strengthen our forward defences”, Mr Stoltenberg said.

In response to Russia “We will enhance our battlegroups in the eastern part of the alliance up to brigade-levels”.

“We will transform the Nato response force and increase the number of our high-readiness forces to well over 300,000,” he said.

Outlining further support for Volodymyr Zelensky’s Ukraine, he said: “Their courage and commitment are an inspiration.

“And I welcome that President Zelensky will join us at the Nato Summit.”

The allies will agree a strengthened comprehensive assistance package for Ukraine, with deliveries of secure communications, anti-drone systems and fuel.

The alliance will also consider Sweden and Finland’s bids to join, applications made in response to the threat posed by Russia.

Turkish leader Recep Tayyip Erdogan has reservations about the plan, but he is due to hold talks with the leaders of the two countries in Madrid.

The Nato meeting will follow the G7 summit in Germany which was also dominated by talks on Ukraine.