The UK is increasingly vulnerable to the threat of missiles and drones after decades of cost-saving cuts eroded its once world-class air defences, military sources and Cold War veterans have warned.

Defence chiefs are understood to be exploring options to regrow Britain's ability to protect critical national infrastructure - like power stations, military bases and government buildings - from the kind of Russian cruise and ballistic missile strikes that are devastating Ukraine. But any credible "integrated air and missile defence" plan will cost billions of pounds and would likely require a further increase in defence spending beyond a proposed rise to 2.5% of national income recently announced by the prime minister, according to defence sources.

"Can the UK defend its cities from the skies if there was a barrage of missiles? No," a senior defence source said, speaking on condition of anonymity. "Do the public know what to do in the event of an air attack? No...

Put simply, are we defended? No." As part of a series called Prepared For War? Sky News visited air defence sites that once played a key role in protecting Britain during the Cold War - and spoke to veterans who were part of the force that had been on alert to respond to any Soviet air threat. Pressing the big red button Flicking a line of switches to prime a simulated batch of missiles from inside a cabin at an old military-base-turned-museum in Norfolk, a former Royal Air Force technician watches a screen as a radar scans for enemy aircraft.

"It's picked up a target," says Robert Findlater, pointing at a dot on the monitor, which looks more like a retro computer game. A beeping noise indicates the signal from the radar is becoming stronger as the hostile aircraft approaches.

Once in range, red letters on one of the screens that had read "hold fire" switch to the words "free to fire.