General Sir Nick Carter will say that the British Army's ability to respond to threats "will be eroded if we don't keep up with our adversaries".
Among the areas of concern Carter's speech highlights are the Russian army's long-range missile strike capability.
The Russian army conducted large scale military exercises previous year, including simulated attacks across northern Europe, from Kaliningrad to Lithuania.
Monday's address in London was officially sanctioned by the Secretary of Defence Gavin Williamson and comes amid widespread speculation of possible defence cuts to the UK's armed forces.
'I believe our ability to pre-empt or respond to these threats will be eroded if we don't match up to them now.
According to Carter, hostile states are being more creating in how they exploit the seams between peace and war.
He will add that that Russian Federation is building an increasingly aggressive expeditionary force, which already boasts capabilities the British Army would struggle to match. Even as Russian Federation was intervening in Syria, it deployed 26 missiles from a 1,500km (930 mile) range.
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.
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This intervention from Carter is as much an appeal for more money to fund the armed forces and to avoid further cuts.
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.
Prime Minister Theresa May is expected to order a national security and defence review to be split in two - the security element proceeding as planned but the defence section put back by months.
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".
When a Strategic Defence and Security Review was carried out in 2015, the MoD said it would cut its civilian personnel from 56,860 to 41,000 by 2020, but figures released earlier this year showed that it still had 56,690 civilian staff, meaning it had only cut 170 posts, rather than the 16,000 it had promised.
Mr Williamson said "hard work" is taking place to give the armed forces the "right resources".
However, some politicians have expressed concerns that the spending levels were not sufficient to meet new threats, and that some of the Ministry of Defence's procurement plans were based on over-optimistic assumptions.