The UK's defence chief of general staff, Sir Nick Carter, has warned that the UK is trailing Russia in terms of defence spending and capability.
Carter used a speech in London to enter publicly into the debate over defence spending, which military chiefs and Conservative MPs claim is down to dangerously low levels.
In case UK cannot keep up with Russia, it will leave the UK exposed especially to unorthodox, hybrid warfare of the kind practised by Russia and other potentially hostile states, according to Carter. Among the biggest threats is cyber-attacks that target both the military and civilian life.
''Our ability to pre-empt or respond to threats will be eroded if we don't keep up with our adversaries,'' said
The Uk ministry of defence is pressing the treasury for a significant increase in spending on the army, navy and air force.
According to Carter, hostile states are being more creating in how they exploit the seams between peace and war. ''We must take notice of what is going on around us or our ability 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. The time to address these threats is now – we cannot afford to sit back.''
The speech, which was approved by defence secretary Gavin Williamson comes amid speculation of potential defence cuts.
The warning follows simulated attacks across northern Europe by Russia.
In the speech, at the Royal United Services Institute today, General Carter highlighted Russia's new cyber warfare capabilities.
Gen Carter also focussed on Russian army's long-range missile strike capability. Even as Russia was intervening in Syria, it deployed 26 missiles from a 1,500km (930 mile) range.