Russian President Vladimir Putin could start hostilities against the West "sooner than we expect", the head of the British Army has said.
Warning of Russia's "eye-watering" military capabilities, Sir Nick Carter laid bare the scale of the threat.
The Chief of the General Staff said the Kremlin was a "clear and present danger" and predicted a conflict would start with something we did not expect, the Daily Mail reports.
"They are not thousands of miles away, they are on Europe's doorstep," he said in a speech at the Royal United Services Institute.
Britain's ability to pre-empt or respond to the threat "will be eroded if we don't match up to them now," he said, adding: "Russia could initiate hostilities sooner than we expect."
Using the dire warning to make the case for more money for the armed forces, General Carter said:
• Britain could scale back its military withdrawal from Germany to allow personnel to race to Eastern Europe if war breaks out;
• Syria's civil war was exploited by Moscow to get its troops combat-ready while testing long-range missiles and other equipment;
• Russia's conventional forces give it a "calculable military advantage";
• Hostile action would be hard to predict and the time to address the threats was now.
General Carter's major speech came as experts issued their own warnings about the threat from Russia and the need for Britain to spend more on defence.
The head of the National Cyber Security Centre warned that a major attack on the UK was a matter of "when, not if".
Ciaran Martin said Britain had been fortunate to avoid a "category one" hacking attack. This is defined as one that could cripple infrastructure such as energy supplies and the financial services sector.
He suggested one was likely in the next two years, telling the Guardian: "It is a matter of when, not if, and we will be fortunate to come to the end of the decade without having to trigger a category one attack."
And the former head of spy agency GCHQ, Robert Hannigan, told the BBC he was seriously concerned about Russia's growing aggression in cyberspace. He said: "It's the single country that's kept me awake, because their intent has changed over the years."
In his speech in London, General Carter said there were stark parallels between the situation before the First World War in 1914 and how Russia might view things now.
He said: "Our generation has become used to wars of choice since the end of the Cold War. But we may not have a choice about conflict with Russia. And we should remember Trotsky's advice that 'you may not be interested in war but war is interested in you'."
Showing a Russian military propaganda video, he said Moscow was developing an "eye-watering quantity of capability".
He cautioned that hostilities would not start with "little green men" – a reference to conventional ground troops in camouflage.
"It will start with something we don't expect. We should not take what we've seen so far as a template for the future," he said.
The Army chief said Russia's doctrine for war uses "all of the instruments of national power, not just the military". He added: "The character of warfare is making it much harder for us to recognise true intentions and distinguish between what is peace and what is war."
He said credible deterrence could be underpinned only by genuine forces and commitment "that earns the respect of potential opponents".
To deter Russia in Eastern Europe, Britain and its Nato allies must improve their speed of recognising what was going on, speed of deciding what to do and speed of assembling forces if needed, he said: "The time to address these threats is now – we cannot afford to sit back."
General Carter said there were no longer two clear and distinct states of peace and war.
He said: "The risk we run in not defining this clearly, and acting accordingly, is that rather like a chronic contagious disease it will creep up on us, and our ability to act will be markedly constrained. And we'll be the losers of this competition."
General Carter said he was "actively examining" keeping supply bases in Germany open once British personnel are brought home.
This would enable troops to return at short notice with equipment already in place. At one point 55,000 UK personnel were stationed in Germany but the latest figure is no more than 4000.
How Russia is using the battlefields of war-torn Syria to test frightening arsenal
Russia has used the battleground in Syria to test a "frightening" range of missiles and other deadly weaponry, the head of the Army warned.
General Sir Nick Carter said Vladimir Putin had been able to develop an "expeditionary capability" for his military in the war-torn country.
The fighting in the Middle East has given large numbers of Russian officers "high-end" war experience.
The Kremlin's military has been able to try out more than 150 new weapons – including long-range missiles – and items of equipment in its war against opponents of Syrian dictator Bashar al-Assad, he said.
In October 2015, Russia launched strikes on Islamic State targets in Syria from warships in the Caspian Sea – about 1500km away.
Russian defence minister Sergei Shoigu said at the time that four warships fired 26 cruise missiles at 11 targets, destroying them and causing no civilian casualties.
General Carter showed a three-minute Russian propaganda video of Moscow's deadly weaponry detailing what he called "eye-watering" capabilities.
The Chief of the General Staff said it was "vital" for Britain to invest in ballistic missile defence, training and getting back to understanding ground-based air defence. At the Royal United Services Institute, he said: "[The Russians] have used Syria to develop an expeditionary capability."
Spelling out specific threats, the Army chief said that Russia was also using large numbers of drones to locate precise targets before firing missiles at them at an extremely rapid rate. The general said that "electronic warfare prompting drone attacks, which then deliver a very frightening array of missiles, is something that is significantly challenging for us".
In just the past five years the number of Russian air, sea and land-based launchers for long-range missiles has increased by a factor of 12, he said, while Moscow has also boasted of increasing the number of missiles with a range of up to 4000km by a factor of 30.
These advances have given the Russians the capability to create mobile "missile domes", which then enable them to "seal airspace over significant distances", the Army chief warned.
General Carter said that these missile domes acted as a "shield in which they can assure their freedom to manoeuvre and deny us the ability to act".
The Russians are also using electronic warfare "at a scale to cue precise targeting by large numbers of drones that enable very accurate and instantaneous fires".
He said that these included "thermobaric warheads", which are fuel-air explosives that can destroy an opponent's forces.