Londoners would only have "a matter of minutes" before being warned of an impending nuclear attack, experts 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.