Britain’s armed forces would struggle to match the Russian army in battle, the head of the Army will say.
General Sir Nick Carter will say the British 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, General Carter will highlight Russia’s new cyber warfare capabilities.
The Russian army conducted a 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 1500km (930 miles) range.
He will add that that Russia is building an increasingly aggressive expeditionary force, which already boasts capabilities the British Army would struggle to match.
Potential military threats to the UK “are now on Europe’s doorstep,” General Carter will say.
Last year Prime Minister Theresa May said Russia had “mounted a sustained campaign of cyber espionage and disruption” against other nations.
‘An appeal to avoid cuts’
By Jonathan Beale, defence correspondent
This intervention from General Carter is as much an appeal for more money to fund the armed forces and to avoid further cuts.
General Carter will say the UK’s ability to respond to threats will be eroded if it doesn’t keep up with its adversaries, and he says the time to address these threats is now.
This appeal is being made with the approval of the defence secretary, Gavin Williamson, who’s made clear he wants more cash from the Treasury.
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.
The speech comes as national security adviser Mark Sedwill conducts a review of the UK’s security capabilities.
There are concerns in the armed forces that the review will prioritise counter-cyber attacks and terrorism, rather than major defence.
Last week Conservative MP Johnny Mercer, a former army officer, 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.