General Sir Nick Carter will stress that Vladimir Putin has built a force that this country would struggle to match.
His address, at the Royal United Services Insititue (RUSI), comes as the British Army is beset by cuts and Putin’s military swells.
General Carter will highlight its long-range missile capability, after Russian troops fired 26 missiles in Syria from a 1,500km (930 mile) range last year.
He will say: “The time to address these threats is now, we cannot afford to sit back.
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“Our ability to pre-empt or respond threats will be eroded if we don’t keep up with our adversaries.
“We must take notice of what is going around us or our ability to take action will be massively constrained.
“Speed of decision making, speed of deployment and modern capability are essential if we wish to provide realistic deterrence.”
Carter is also set to highlight Russia’s capabilities in the field of cyber warfare.
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“The threats we face are not thousands of miles away but are now on Europe’s doorstep”
General Sir Nick Carter
He will add: “State-based competition is now being employed in more novel and increasingly integrated ways and we must be ready to deal with them.
“The threats we face are not thousands of miles away but are now on Europe’s doorstep – we have seen how cyber warfare can be waged both on the battlefield and to disrupt normal people’s lives.
“We in the UK are not immune from that.”
Just last week, RAF fighter jets were deployed to see off two Russian supersonic heavy bombers less than 35 miles from British airspace.
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Two 1,550mph Typhoon fighter jets were sent to escort the Russian planes as they flew over the North Sea off the coast of northern Scotland.
Russia also held joint war drills involving almost 13,000 troops, 70 jets, helicopters and 250 tanks with Belarus armed forces last year.
NATO feared the drills could be seen as “serious preparation for a big war” and sent around 4,000 troops to the Baltics and Poland.
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The Kremlin claimed the exercises were defensive in nature and said they are “nothing out of the ordinary”.
General Carter will say: “The traditional threat still remains and last year we saw Russia undertake simulated attacks across northern Europe.
"We must also look closely at how countries are now being more creative in how they exploit the seams between peace and war.”