THERESA May has warned Britain could withhold tens of millions of pounds from the United Nations –unless the organisation can transform itself into something more "agile, transparent and joined up".
The Prime Minister delivered a clear message in her keynote speech to the UN General Assembly, voicing frustration at how billions of pounds is squandered on bureaucracy, botched aid projects and other schemes.
She spoke fondly of the UN's overall aims of international peace and co-operation, calling it an "amplifying force" that enables nations to "live up to these standards in word, spirit and deed, to our collective and individual benefit".
But she said unless sweeping reforms were made, the UK Goverment is willing to cut Britain's annual £90million basic UN membership contribution by nearly a third.
She also made a point of praising the Paris climate accord and called on all countries to "come together and defend" the rules-based system of international agreements.
Her comments will be seen as a rebuke to US President Donald Trump, who has threatened to withdraw America from the historic pact, and who she was due to meet after making her speech.
"If this system we have created is found no longer to be capable of meeting the challenges of our time – then there will be a crisis of faith in multilateralism and global co-operation that will damage the interests of all our peoples," she said.
"So those of us who hold true to our shared values - who hold true to that desire to defend the rules and high standards that have shaped and protected the world we live in - need to strive harder than ever to show that institutions like this United Nations can work for the countries that formed them, and for the people who we represent.
"This means reforming our United Nations and the wider international system so it can prove its worth in helping us to meet the challenges of the 21st Century."
"But we should also acknowledge that throughout its history the UN has suffered from a seemingly unbridgeable gap between the nobility of its purposes and the effectiveness of its delivery," she said.
"When the need for multilateral action has never been greater, the shortcomings of the UN and its institutions risk undermining the confidence of States as members and donors. Even more importantly, they risk the confidence and faith of those who rely upon the blue helmets; who rely upon that sign I stand in front of today coming to their aid in the darkest of hours."
Mrs May said the UN High Commissioner for Refugees and UNICEF, which helps children in danger, deserved praise for "vital missions" around the world.
Mrs May urged world leaders to support a reform plan to "create a more agile, transparent and joined up organisation".
Mrs May vowed that Britain would continue giving billions in overseas aid to UN projects and contribute troops to UN peacekeeping operations around the world.
But she added: "In turn the UN and its agencies must win our trust by proving to us and the people we represent that they can deliver.
"That is why we will remain generous in our funding but set aside 30 per cent to be paid only to those parts of the UN that achieve sufficient results."
Her intervention followed criticism of the UN by US President Donald Trump - who the Prime Minister had stealthily reproached earlier in her speech.
Without mentioning Mr Trump or the US by name, Mrs May made clear that she regards it as vital that all UN states stick by their commitments in agreements to tackle issues ranging from security to trade protectionism and climate change.
"I believe that the only way for us to respond to this vast array of challenges is to come together and defend the international order that we have worked so hard to create and the values by which we stand," she said.
"For it is the fundamental values that we share - values of fairness, justice and human rights -that have created the common cause between nations to act together in our shared interest and form the multilateral system.
"And it is this rules-based system which we have developed - including the institutions, the international frameworks of free and fair trade; agreements such as the Paris Climate Change accord and laws and conventions like the Non-Proliferation Treaty - which enables the global co-operation through which we can protect those values."
"On migration, our starting point must be that it can benefit both countries and migrants themselves - but only when it is safe, orderly, well managed and legal.
"If we do not manage this effectively, we will fail both our own citizens and those taking these dangerous journeys.
"And we will push more people into the curse of modern slavery and the hands of the human traffickers and organised criminal groups that drive this inhuman industry."
The UN needed to do more to encourage private-sector enterprise to help with aid projects to help lift developing nations out of poverty.
"For the truth is that despite our best efforts, we are not succeeding. We must do more," she said.