The head of the British Army will warn that the country's government risks increasing its vulnerability to Russian aggression if it fails to invest further in its military infrastructure.
Gen. Nick Carter is expected to tell the Royal United Services Institute on Monday that Britain will be unable to preempt or respond to adversaries unless its military capabilities are overhauled.
Carter will warn that the UK would struggle to match Russia's military capabilities, citing Russia's long-range missile strikes on Syria, where 26 missiles were deployed from a 1,500 km (930 mile) range.
He will also highlight Russia's simulated attack exercises across northern Europe, from Kaliningrad to Lithuania, as part of a new tactic in exploiting the gray areas between peace and war.
"Our ability to preempt or respond to threats will be eroded if we don't keep up with our adversaries," Carter is expected to say.
"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 both waged on the battlefield and to disrupt normal people's lives; we in the UK are not immune from that.
"We must take notice of what is going on 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.
"The time to address these threats is now. We cannot afford to sit back."
The speech, which comes at a time when there is speculation over cuts to defense spending, will once again shine a light on tensions between Britain and Russia.
It also comes a month after Britain's Air Chief Marshal Stuart Peach warned that the country should prioritize protecting undersea communication cables from Moscow, adding their disruption could be "potentially catastrophic" to the economy.
Last month, UK Foreign Secretary Boris Johnson and his Russian counterpart Sergey Lavrov admitted significant problems in their countries' relationship.
Johnson said the "difficulties" could not be ignored but that the two nations "have to find a way forward," while Lavrov said it was "not a secret" that the relationship was at a "very low point."
In November, British Prime Minister Theresa May launched a fierce attack on Russia, accusing it of interfering in elections and planting fake news stories in a bid to undermine Western institutions.
May accused Moscow of attempting to "weaponize information" as part of a "sustained campaign of cyberespionage and disruption." Russia's actions were "threatening the international order," she said.