The United Nations secretary general, António Guterres, will warn on Saturday of the peril posed by “powerful forces lining up to undermine global cooperation” in an address to mark the 80th anniversary of the UN’s first major meeting.

Speaking in London’s Methodist Central Hall – the site where eight decades earlier delegates from 51 countries came together for the inaugural session of the general assembly – the UN head will make an impassioned plea for the virtues of multilateralism and international law to prevail during a period of deepening global uncertainty.

In January 1946, the general assembly’s first resolution focused on disarmament and the elimination of atomic weapons as a global goal.

Now, Guterres warns of a planet facing myriad threats that were then unthinkable, citing the climate crisis and threat from “cyberspace” at a time many countries are locked into a new arms race, though he will hold off from naming offending states.

The secretary general, who will step down at the end of 2026, will say: “Last year, global military spending reached $2.7tn – over 200 times the UK’s current aid budget, or equivalent to over 70% of Britain’s entire economy.”

The deepening investment in weaponry runs alongside his concerns that the drivers of climate breakdown are being wilfully ignored and online content is undermining democracy.

“As the planet broke heat records, fossil fuel profits continued to surge. And in cyberspace, algorithms rewarded falsehoods, fuelled hatred, and provided authoritarians with powerful tools of control,” he will tell the London audience.

Comments from the 76-year-old come at a time of chronic funding difficulties for the UN, largely driven by the decisions of the US president, Donald Trump.

The US has announced it would be allocating just $2bn (£1.5bn) to UN humanitarian assistance, a fraction of its previous contributions as the leading funder. The announcement came with a warning from the US state department that the global body must “adapt, shrink or die”, and that demands would be imposed on countries receiving the money.

Just over a week later, Trump announced the US withdrawal from multiple UN agencies as well as its key climate treaty.

Experts say the funding cut will lead to a shrunken, less effective international aid system, with the UN already saying a funding shortfall threatens to cripple its global peacekeeping operations.

Guterres, however, says reforms will ensure the “United Nations is more agile, more coordinated and more responsive”.