British troops are being warned they must be prepared to fight to prevent the spread of war in Europe.
General Sir Patrick Sanders, Chief of the General Staff, will say in a speech on Tuesday that he had never seen such a clear threat to peace and democracy as the "brutal aggression" of Russian president Vladimir Putin.
The new Army head will say Britain must “act rapidly” to ensure the war in Ukraine is not drawn into a full-scale conflict.
He will address a conference organised by the Royal United Services Institute think tank after writing to all the troops under his command telling them they must prepare "to fight in Europe once again".
At the same event, Defence Secretary Ben Wallace is expected to issue a fresh call for increased defence spending.
His call comes as Prime Minister Boris Johnson prepares to join other Nato leaders in Madrid for a summit at which they are expected to agree the biggest overhaul of the Western military alliance since the end of the Cold War.
In his first public engagement since taking up his post, General Sanders is expected to say: "In all my years in uniform, I haven’t known such a clear threat to the principles of sovereignty and democracy, and the freedom to live without fear of violence, as the brutal aggression of president Putin and his expansionist ambitions.
"This is our 1937 moment. We are not at war - but must act rapidly so that we aren’t drawn into one through a failure to contain territorial expansion.
"I will do everything in my power to ensure that the British Army plays its part in averting war."
Despite the recent emphasis on new capabilities, such as drones and cyber warfare, he will say that land forces will remain crucial in any conflict, adding: "You can’t cyber your way across a river."
General Sanders is expected to say more of the Army needs to be ready “more of the time", from "the general in (Ministry of Defence) Main Building, to the young lance corporal in the barrack room, from the reservist on a weekend exercise, to the civil servant in Army headquarters".
In March the Defence Secretary wrote to Chancellor Rishi Sunak ahead of his spring statement warning UK defence spending was set to drop below the Nato minimum of two percent of GDP by the middle of the decade unless the Treasury committed more resources.