Britain’s armed forces risk falling behind Russia without more investment, the head of the Army will say.
General Sir Nick Carter will say the Army’s ability to respond to threats “will be eroded if we don’t keep up with our adversaries”.
The speech – approved by Defence Secretary Gavin Williamson – comes amid speculation of potential defence cuts.
The warning comes after Russia practised simulated attacks across northern Europe.
In the speech, which will take place at the Royal United Services Institute on Monday, Gen Carter will highlight Russia’s new cyber warfare capabilities.
The Russian army conducted large scale military exercises last year, including simulated attacks across northern Europe, from Kaliningrad to Lithuania.
Gen Carter will also highlight the Russian army’s long-range missile strike capability. While Russian forces were intervening in Syria, 26 missiles were deployed from a 1,500km (930 mile) range.
He will add that Russia is building an increasingly aggressive expeditionary force, which already boasts capabilities the Army would struggle to match.
Potential military threats to the UK “are now on Europe’s doorstep,” Gen Carter will say.
Prime Minister Theresa May’s spokesman said the UK’s £36bn annual defence budget meant the government was addressing the range of threats from “a position of strength”.
Labour shadow defence secretary Nia Griffith said she was sure Gen Carter would have tried to make his case behind closed doors but had resorted to going public as a “last resort”.
She added that if voicing concerns over funding made the prime minister and the chancellor “wake up”, it would have been worth “making the fuss”.
Admiral Lord West, former head of the Royal Navy, said Gen Carter’s warnings were unprecedented.
He told the BBC: “Certainly it’s extremely unusual for a serving chief to even talk about threats.
“But for them to talk about the need for more resources, I haven’t known that in my 52 years in the navy – I mean this is extraordinary.”
In December Air Chief Marshal Sir Stuart Peach said the UK should prioritise protecting undersea cables from the Kremlin, as disruption could be “potentially catastrophic” to the economy.
Last year Mrs May said Russia had “mounted a sustained campaign of cyber espionage and disruption” against other nations.
Robert Hannigan, who stood down as director of the intelligence monitoring service GCHQ last year, said Russia was the single country that had “kept me awake” in recent years.
He told BBC Radio 5 live: “They have always been very capable, but in the last two, three, four years they’ve become quite aggressive.”
Gen Carter’s speech comes as national security adviser Mark Sedwill conducts a review of the UK’s security capabilities.
All three armed forces have been asked to put forward options for cuts as the Ministry of Defence looks at how it will control spending.
There are concerns within the forces that the review will prioritise counter-cyber attacks and terrorism, rather than major defence.
Colonel Rupert Wieloch, a former senior British commander of forces in Libya, said the structure of funding needed to be addressed so that money for the armed forces was not “confused” with money for cyber security.
He told the BBC: “I think we need to get back to the state where the money that goes to the armed forces is ring-fenced and not allowed to be taken away for issues at home such as cyber (security).”
Britain’s armed forces are now at their smallest since the Napoleonic wars. The size of the Army has been cut from more than 100,000 to 82,000 since 2010.
Last week Conservative MP Julian Lewis, chairman of the Commons defence select committee, posed an urgent question in the Commons after speculation that there were plans to cut the UK military by 14,000 service personnel, nine warships and 100 helicopters.
Mr Williamson said “hard work” is taking place to give the armed forces the “right resources”.
Some MPs have called to increase defence spending to 3% of GDP – it is currently at 2%, in line with the guideline for Nato members.