Nato is set to massively increase its available forces on high alert to more than 300,000 troops in the most significant overhaul of the alliance’s defence strategy since the Cold War.
In a historic policy shift, Jens Stoltenberg, its secretary-general, said the alliance would also genuinely fortify the border with Russia to fully defend allied territory in a response to Moscow’s invasion of Ukraine.
“We will transform the Nato response force and increase the number of our high readiness forces to well over 300,000,” he told reporters ahead of a crunch summit in Madrid.
“Russia has walked away from the partnership and the dialogue that Nato has tried to establish with Russia for many years,” Mr Stoltenberg added.
“They have chosen confrontation instead of dialogue. We regret that – but of course, then we need to respond to that reality.”
As part of the pledge, Boris Johnson will announce “significant reinforcement” for Estonia, potentially more than doubling the number of British troops available to the Baltic State.
The move is more than a sevenfold increase to Nato’s current rapid response force of 40,000 troops, which alliance members have largely lost trust in.
It will include several new structures under which Nato allies would commit ships, warplanes and troops to be ready to deploy across the alliance’s eastern flank with as little as 24 hours' notice.
The Telegraph understands an initial 44,000 troops will be placed under the high-readiness category – as part of Nato’s so-called New Force Model, to bolster defences on the eastern flank – and will be available with less than 15 days' notice.
While not all 300,000 troops will be deployed, Nato’s top military commander would know exactly which forces are at their disposal and how quickly they could enter the battlefield.
The availability list does not include US troops after Washington refused to make an initial commitment in order to pressure European allies into offering up more resources.
At their summit starting in Madrid on Tuesday, Nato leaders are expected to set out in more detail the amount and types of forces they are willing to commit to the new high-readiness model.
Britain is expected to announce that as many as 5,000 troops could be made available to reinforce Estonia, where 1,650 British soldiers are already deployed.
Mr Stoltenberg confirmed Nato units deployed across eight eastern and south-eastern alliance members would be strengthened to brigades, normally comprising around 3,000 to 5,000 troops.
Nato officials on Monday said the transition to the new model would not be completed until next year as details were still being hammered out.
The new military plan was announced following a warning from the Baltic States that they would be “wiped off the map” in the event of a Russian invasion under Nato’s existing strategy.
The doctrine foresees Lithuania, Estonia or Latvia falling to Russia before being liberated in a Nato counter-offensive after 180 days.
Nato will scrap its traditional “tripwire” defence strategy in Madrid this week following concerns it is no longer viable.
Ben Wallace, the Defence Secretary, recently told Nato allies that the “tripwire doctrine was not really up to what we have seen happen in Ukraine”.
“The first fight is the most important,” he said at a meeting of Nato defence ministers.
The defining Nato summit comes as diplomatic tensions simmer over freight shipments to Kaliningrad.
Lithuania was on the receiving end of significant threats from Moscow after it blocked Russia from using railways to transport coal, metals and other sanctioned goods to the military exclave.
In a direct warning to the Kremlin, Mr Stoltenberg said: “I’m confident that Moscow, President Putin understands our collective security guarantees, understands the consequence of attacking a Nato allied country.
“It will trigger a response from the whole Alliance.”
At the summit, Nato will also drastically change its outlook on Russia from the current description of Moscow as a strategic partner.
Russia would be designated as the “most direct and immediate threat to our security,” Mr Stoltenberg said of the language due to be adopted as part of the “Strategic Concept” – Nato’s plan for the next decade.
The secretary-general poured cold water on hopes of Sweden and Finland being admitted to the alliance at the summit.
He said he would host talks between Turkey and the Nordic nations in Madrid, but warned against an immediate breakthrough on Ankara’s opposition to them joining.
“I will not make any promises or speculate about any specific timelines. The summit has never been a deadline,” Mr Stoltenberg said.
To further tighten European security, the Western military alliance will announce plans to run training missions to bring Ukraine’s armed forces to Nato-standard.
British and US officials have raised concerns that, while vital, weapons shipments will not be enough to guarantee Kyiv’s safety in the future.