Sir Nick Carter said Britain “cannot afford to sit back” as Vladimir Putin strengthens his armed forces.
Speaking at the Royal United Services Institute (RUSI) he said the “threats” posed by Russia needed to be addressed “now”.
He said: “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.
“The time to address these threats is now – we cannot afford to sit back.”
He said Russian hostilities could come “sooner than we expect” and won’t simply be a case of “little green men – it will start with something we don’t expect”.
Earlier today Prime Minister Theresa May said: “Looking at the speech, the Chief of General Staff is saying that we face a range of threats and we need to make sure we have the capabilities required to address them.
“That is exactly what we are doing as part of the National Security Capability Review, and we are doing that from a position of strength where we have a £36 billion defence budget which will rise to almost £40 billion by 2020/21.
“That’s the biggest defence budget in Europe, the second biggest in Nato and the fifth largest in the world.”
This is a developing story, more to follow…