Londoners would only have “a matter of minutes” before being warned of an impending nuclear attack, experts have said, with one bomb in today’s arsenal enough to wipe out the entire city.

It will be 75 years this week since the atomic bombings of Hiroshima and Nagasaki, which began the period in history that came to be known as the nuclear age.

If an intercontinental ballistic missile (ICBM) were to head to London, the warning time would be measured in minutes, not hours, said Dr Lyndon Burford, a post-doctoral research associate in the Centre for Science and Security Studies at King’s College London.

“The Prime Minister would have minutes to make it to a safety bunker,” he said.

As for the public, Dr Burford said he is sceptical of the UK Government’s assertion that it could respond to a single nuclear use in an urban area, citing a warning given by the International Committee of the Red Cross (ICRC) in 2013 that, as things stand, “there is no effective way of delivering humanitarian assistance to victims of a nuclear blast”.

Dr Burford added: “We have learnt that ionising radiation has a gendered impact – women and girls are disproportionately impacted by the negative effects. We don’t know why, but that’s what the science says.”

The nuclear weapons that exist in today’s arsenal are “much more powerful” than the ones used to bomb Hiroshima and Nagasaki, said Matt Korda, a research associate for the nuclear information project at the Federation of American Scientists (FAS).

The warheads that bombed the Japanese cities on August 6 and 9 in 1945, nicknamed Little Boy and Fat Man, achieved blasts of around 15-20 kilotons.

The first US atomic bombing killed 140,000 people in Hiroshima, while the second atomic attack on Nagasaki killed another 70,000.

“By contrast, today’s weapons can achieve yields of several hundred, sometimes over 1,000, kilotons, due to the introduction of multi-stage thermonuclear weapon designs during the early years of the Cold War,” Dr Korda added.

“A nuclear detonation of several hundred kilotons over the centre of London would destroy most of the city, and could break windows as far away as Croydon and Walthamstow.”

According to the FAS, the number of nuclear weapons in the world has declined significantly since the Cold War, down from a peak of around 70,300 in 1986 to an estimated 13,410 in early 2020.

The FAS estimates that around 91% of all nuclear warheads are owned by Russia and the United States, which each have around 4,000 warheads in their military stockpiles.

The military theory of mutually assured destruction (MAD), in which a nuclear attack by one superpower would be met with an overwhelming nuclear counter-attack, thus acting as a deterrent to nuclear warfare, is a hotly debated topic.

Dr Burford said: “Many former military and governmental experts are increasingly pointing out that, even if deterrence did work in some instances – and we can never ‘know’ in the scientific sense, because deterrence is an internal, psychological process – in other cases, it is clear that luck played a significant role in preventing the use of nuclear weapons.

“So, did MAD work? Only if we accept that at a minimum, sometimes we just have to leave it to luck to determine whether or not we have a nuclear war.”

Professor Malcolm Chalmers, deputy director-general at the Royal United Services Institute, said the use of nuclear weapons against the UK is most likely to take place during a time of extreme national crisis or war.

“A deliberate ‘bolt from the blue’ attack cannot be ruled out altogether, but it is extremely hard to see what an enemy could hope to achieve,” he said.