Patrick Sanders, a general who became the army's top officer this month, said Moscow's attack on its neighbour represented "a clear and present danger" to Western "principles of sovereignty and democracy".

In a wide-ranging speech to mark taking over the role, he argued Britain's armed forces must react by accelerating modernisation plans and immediately increasing battle readiness alongside NATO allies.

"This is our 1937 moment," Sanders told an audience at the Royal United Services Institute (RUSI) military think-tank, referring to Western allies' struggle to subdue Nazi Germany's aggression in the run-up to World War II.

"We're not at war. We must act rapidly so that we aren't drawn into one through a failure to contain territorial expansion."

Adding it is "dangerous" to assume Russian aggression will end with Ukraine, he predicted Moscow will pose "an even greater threat to European security after Ukraine than it was before".

"The Russian invasion has reminded us of that time-honoured maxim that if you want to avert conflict, you better be prepared to fight," Sanders said.

His comments come as NATO members begin to meet in Madrid Tuesday for a summit, and follow the military alliance announcing it will boost its high-readiness force from 40,000 to 300,000 troops.

Its chief Jens Stoltenberg has called the move "the biggest overhaul of our collective defence and deterrence since the Cold War".

Sanders said the gathering in the Spanish capital was an opportunity for Britain to demonstrate its "enduring commitment to our allies" and "lead by example" in mobilising the army.

He noted mobilisation would now be the army's "main effort" over the coming years, "to help prevent war in Europe by being ready to fight and win alongside our NATO allies".

"It will be hard work, a generational effort and I expect all ranks to get ready, train hard and engage," he added.

The army head also backed the US and UK governments' stance of aggressively arming Ukraine in its fight against Russia.

"This is the moment to defend the democratic values that define us," Sanders said.

"This is the moment to help our brave Ukrainian allies in their gallant struggle.

"This is the moment we stand with our friends and partners to maintain peace throughout the rest of Europe."