In a keynote speech to be delivered on Saturday in London's Methodist Central Hall — the venue where delegates from 51 nations convened in 1946 for the organisation's inaugural general assembly — Guterres will urge world leaders to defend multilateralism and uphold international law at a time of mounting geopolitical instability.

Recalling the UN's earliest days, Guterres notes that the organisation's first resolution focused on nuclear disarmament and the elimination of atomic weapons. Eight decades on, he argues, the world confronts a vastly more complex set of threats.

The secretary general is expected to point to accelerating climate breakdown, the rise of digital and cyber risks, and a renewed global arms race, though he will avoid singling out individual countries.

Global military spending reached an estimated $2.7 trillion last year, according to Guterres — a figure he contrasts sharply with investment in development and humanitarian aid. He will note that the sum is more than 200 times the United Kingdom's current aid budget and equivalent to over 70% of the country's entire economy.

The surge in defence spending, he warns, is occurring alongside a continued failure to address the root causes of climate change and the growing influence of online platforms in distorting democratic processes.

"As record temperatures were shattered, profits from fossil fuels continued to rise," Guterres will say. "At the same time, digital systems rewarded misinformation, amplified hatred and handed new tools of control to authoritarian actors."

The remarks come as the UN faces one of the most severe funding crises in its history, largely driven by policy decisions in Washington. The administration of US president Donald Trump has sharply reduced financial support for UN humanitarian operations, pledging just $2 billion — a fraction of previous contributions.

The funding announcement was accompanied by warnings from the US State Department that the UN must dramatically reform, with conditions attached to future assistance. Shortly afterwards, Washington confirmed its withdrawal from several UN agencies as well as from a key international climate treaty.

Analysts say the cuts risk weakening the global aid system and undermining peacekeeping missions, with UN officials already cautioning that a growing budget shortfall could severely limit operations in conflict zones.

Despite the pressure, Guterres insists the organisation can adapt. He argues that ongoing reforms will make the UN more flexible, better coordinated and capable of responding to global crises.

As he approaches the end of his term in 2026, the secretary general frames the anniversary not as a moment for nostalgia, but as a warning — and a call to defend the principles on which the UN was founded.