The warning comes as 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.
General Sir Patrick Sanders, the chief of the general staff, is expected to tell the annual army conference on Tuesday he will focus on mobilising the army to prevent the spread of war in Europe by being “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.”
In a reference to the start of the First War World, General Sanders will say that “this is not the rush to war at the speed of the railway timetables of 1914”.
Defence secretary Ben Wallace is expected to share similar sentiments at the same event. Mr Wallace is set to issue a fresh call for increased defence spending in the years ahead to counter the growing threat.
A defence source did not deny reports that Mr Wallace asked Mr Johnson in a letter to lift the annual military budget from the current Nato minimum target of 2 per cent of GDP to 2.5 per cent by 2028.
“We do not comment on alleged leaks. The defence secretary and the prime minister have always said that the government will respond to any changes in threat which is why in 2020 the Ministry of Defence received a record defence settlement,” they said.
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 2 per cent of GDP by the middle of the decade unless the Treasury committed more resources.