The United Nations is at risk of "imminent financial collapse" due to member states not paying their fees, the body's head has warned.

António Guterres said the UN faced a financial crisis which was "deepening, threatening programme delivery", and that money could run out by July.

He wrote in a letter to ambassadors that all 193 member states had to honour their mandatory payments or fundamentally overhaul the organisation's financial rules to avoid collapse.

It comes after the UN's largest contributor, the US, refused to contribute to its regular and peacekeeping budgets, and withdrew from several agencies it called a "waste of taxpayer dollars".

Guterres said the UN had faced financial crises in the past but that the current situation was "categorically different".

"Decisions not to honour assessed contributions that finance a significant share of the approved regular budget have now been formally announced," the secretary general wrote, without naming specific members.

He said the "integrity of the entire system" depended on states adhering to their legal obligation under the UN charter to pay their "assessed contributions", adding that 2025 ended with a record amount unpaid - equivalent to 77% of the total owed.

Guterres said a rule that the UN must return unspent money to members if it could not implement a budget created a "double blow" in which it was "expected to give back cash that does not exist".

"I cannot overstate the urgency of the situation we now face. We cannot execute budgets with uncollected funds, nor return funds we never received."

He wrote: "The bottom line is clear: either all member states honour their obligations to pay in full and on time – or member states must fundamentally overhaul our financial rules to prevent an imminent financial collapse."

The US is the international organisation's largest contributor, but President Donald Trump has said it was not fulfilling its "great potential" and has criticised it for failing to support US-led peace efforts.

In January, he withdrew the US from dozens of international organisations, including some 31 UN agencies, to "end American taxpayer funding and involvement in entities that advance globalist agendas over US priorities".

In late December, the US pledged $2bn (£1.5bn) to fund UN humanitarian programmes - while warning at the same time the organisation must "adapt or die".

This was a fraction of its previous spending. In 2022, its contribution to the UN's humanitarian work was estimated at $17bn.

Guterres had warned earlier that same month that the UN faced its most fragile financial position in years - again citing unpaid fees - having said in October that it faced a "race to bankruptcy".

Trump has separately been accused by critics of seeking to replace some functions of the UN with his Board of Peace to oversee regeneration efforts in Gaza.

The US president has said its work would happen "in conjunction with the United Nations" - but when previously asked by a Fox TV journalist whether the board would take the UN's place, he replied: "Well, it might."