The UK faces “proximate, rapidly evolving” threats which could prove to be “existential” if left unchecked, a military chief has warned.

Air chief marshal Sir Harv Smyth said that the “international rules-based order”, which long secured Britain’s “sanctuary and prosperity” is being “actively challenged and dismantled by our adversaries.”

“The threats we face are no longer distant storm clouds on the far horizon”, Sir Harv said as he delivered the Lord Trenchard Memorial Lecture at the Royal United Services Institute on Monday.

“They are proximate. They are rapidly evolving.…