Video report by ITV News Correspondent Paul Davies
The head of the British Army has warned the country's ability to respond to Russian threats will erode if the UK does not keep up with its enemies.
General Sir Nick Carter said Moscow is building an increasingly aggressive and expeditionary force that already boasts capabilities that outmatch UK forces.
It comes amid widespread speculation about possible defence cuts to personnel and equipment amid a review of the UK's security capabilities.
Defence Secretary Gavin Williamson approved General Carter's speech to the Royal United Services Institute as he bids to put pressure on the Treasury for increased funding.
Mr Williamson last week claimed "hard work" is taking place across Government to give the "right resources" to the armed forces.
There have been calls to increase defence spending to 3% of GDP from some MPs, and reports there are plans to cut the armed forces' strength by more than 14,000, as well as the combination of elite units of paratroopers and Royal Marines to save cash.
General Carter used his speech to highlight how last year Russia undertook simulated attacks across Northern Europe.
During his speech General Carter stressed that Britain "must take notice of what is going on around us" or that the ability by the UK to take action will be "massively constrained".
"Speed of decision making, speed of deployment and modern capability are essential if we wish to provide realistic deterrence," he added.
"The time to address these threats is now - we cannot afford to sit back."
"Our ability to pre-empt or respond to threats will be eroded if we don't keep up with our adversaries.
"State-based competition is now being employed in more novel and increasingly integrated ways and we must be ready to deal with them.
"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. We in the UK are not immune from that."
Last month the chief of the defence staff Air Chief Marshal Sir Stuart Peach also addressed the threat of Russia.
He said the UK's military has prioritised the protection of undersea cables from the Kremlin warning if they are cut or disrupted there would be an immediate and "potentially catastrophic" hit to the economy.
And using her address at the Lord Mayor's Banquet in London, Prime Minister Theresa May also said last year that Russia had "mounted a sustained campaign of cyber espionage and disruption" against other countries.