Britain's armed forces must be prepared to "fight and win" to prevent the spread of war in Europe, the new head of the British Army is warning.
In a speech on Tuesday, Gen Sir Patrick Sanders, the Chief of the General Staff, will say he had never seen such a clear threat to peace and democracy as the "brutal aggression" of Russian President Vladimir Putin.
Gen Sanders will liken the current situation to the run up to the Second World War, saying Britain must be prepared to "act rapidly" to ensure it is not drawn into a full-scale conflict through its failure to contain Russian expansionism.
His latest warning, in an address to a conference organised by the Royal United Services Institute think tank, comes after he wrote to all of the troops under his command, telling them they must prepare "to fight in Europe once again".
Speaking at the same event, UK Defence Secretary Ben Wallace is expected to issue a new call for increased defence spending in the years ahead to counter the growing threat.
In his first public engagement since taking up his post, Gen Sanders will say his focus is on preventing the spread of war in Europe by having the army "ready to fight and win alongside our Nato allies and partners".
"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," he is expected to say.
"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."
Traditional warfare not anachronistic
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.
"You can't cyber your way across a river."
In a reference to the start of the First World War, Gen Sanders will say that "this is not the rush to war at the speed of the railway timetables of 1914".
Instead he will say deterring Russia means "more of the army 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".
Mr Wallace will repeat his call for increased investment in defence to meet the changing international environment.
In March, he wrote to UK Chancellor Rishi Sunak before his spring statement, warning UK defence spending was set to drop below the Nato minimum of 2 per cent of GDP by the middle of the decade unless the Treasury committed more resources.
"The defence secretary is expected to emphasise that now that the threat has changed, governments must be prepared to invest to keep us safe," a PA defence source said.