General Sir Nick Carter, the chief of the general staff, warned in a speech at the Royal United Services Institute think tank in London that Russian Federation could challenge Britain and its North Atlantic Treaty Organisation allies "sooner than we expect".
Speaking at the Royal United Services Institute (RUSI) on 22 January, Carter warned that the UK must be prepared for a pre-emptive attack from Russian Federation "sooner than expected".
The speech, which the new Defense Secretary Gavin Williamson has approved, is the latest of a wider pushback against military cuts from defense officials.
General Sir Nick Carter will directly compare Britain's military to Russia's and say there are areas where the UK is worse off in a speech at the Royal United Services Institute today.
"Our ability to pre-empt or respond to threats will be eroded if we don't keep up with our adversaries", he will tell the London-based think tank.
"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".
Emphasising the mounting threat posed by Russia's conventional and unconventional military build-up, Carter used a Russian military propaganda video to demonstrate the volume and scope of recent Russian acquisitions.
Traditional threats still remain, Gen Carter will say, in reference to the Russian army conducting grand military exercises including simulated attacks past year in Northern Europe.
Gen Carter also focussed on Russian army's long-range missile strike capability.
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The UK now spends more on defence than any other European nation, paying out £36billion previous year compared to France's £32billion.
Defence Minister Gavin Williamson, who only took over the role in November after his predecessor Michael Fallon stepped down over a sexual harassment scandal, is reportedly calling for increased funding.
"We must take notice of what is going on around us or our ability to take action will be massively constrained".
A security review now being carried out by National Security Adviser Mark Sedwill is expected to be published within weeks.
"Speed of decision making, speed of deployment and modern capability are essential if we wish to provide realistic deterrence", he will add.
The UK spends 2% of GDP on defence, according to The Guardian, but the Ministry of Defence wants that figure to rise to at least 3%.
Williamson made the defence budget a priority since taking office past year.
The peer, who has also served as a United Kingdom representative to North Atlantic Treaty Organisation, acknowledged France and the United Kingdom have upped their commitment to cooperation on counter-terror and cyber-security initiatives in recent years "in response to recent terrorist attacks".