Mr Johnson is expected to announce “significant reinforcement” for Estonia, possibly more than doubling the number of British troops available to the Baltic state.
Mr Stoltenberg also confirmed that NATO units deployed across eight eastern and south-eastern alliance members would be reinforced to brigades, which normally consist of about 3,000 to 5,000 troops.
NATO officials said on Monday that the transition to the new model would not be completed until next year, with details still being hammered out.
The defining NATO summit comes as diplomatic tensions simmer over cargo shipments to Kaliningrad. Lithuania has received threats from Moscow for stopping Russia from using railways to transport coal, metals and other goods approved to the military exclave.
Mr Stoltenberg said: “I am confident that Moscow, President Putin, understands our collective security guarantees, understands the consequences of the attack on a NATO allied country. It will provoke a reaction from the whole alliance. “
NATO will also change its outlook on Russia from the current description of Moscow as a strategic partner. Russia will be identified as the “most direct and immediate threat to our security”, Mr Stoltenberg said of the language to be adopted as part of the “Strategic Draft” – NATO’s plan for the next decade.
The secretary general poured cold water on the hope that Sweden and Finland would be admitted to the alliance at the summit. He said he would hold talks between Turkey and the Nordic nations in Madrid, but warned of an immediate breakthrough on Ankara’s opposition to them joining.
NATO will also announce plans to conduct training missions to bring Ukraine’s armed forces up to the alliance’s standard.