British Prime Minister Liz Truss used her speech at the UN General Assembly to warn democracies that they must deliver for their citizens on safety and the economy or they will “fall behind” autocracies.
Democratic societies face a “real struggle” against authoritarianism and governments cannot be complacent, she told world leaders.
She invoked the queen’s speech to the annual general assembly in 1957 on delivering on the ideals of sovereignty and human rights.
“Now we must show that will. We must fight to defend those ideals. And we must deliver on them for all our people,” Ms Truss said.
Ms Truss said the UK’s strength came “from the strong foundations of freedom and democracy”, while “autocracies sow the seeds of their own demise by suppressing their citizens”.
“Autocracies are fundamentally rigid and unable to adapt," an advance copy of her speech said.
"Any short-term gains are eroded in the long term because these societies stifle the aspiration and creativity which are vital to long-term growth. But we cannot simply assume there will be a democratic future.
“There is a real struggle going on between different forms of society – between democracies and autocracies.
"Unless democratic societies deliver on the economy and security our citizens expect, we will fall behind.
“We need to keep improving and renewing what we do for the new era, demonstrating that democracy delivers.”
Mr Putin has announced that referendums on Russian annexation will be held in areas of Ukraine under Moscow's control. World leaders have decried the move.
Ms Truss was set to accuse Mr Putin of “desperately trying to claim the mantle of democracy for a regime without human rights or freedoms”.
She said progress as a “new Britain for a new era” begins with economic growth and building an economy that “rewards enterprise and attracts investment”.
But it also means “securing affordable and reliable supplies of energy” and “cutting off the toxic power and pipelines from authoritarian regimes”.
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Ms Truss praised the “strength of collective purpose” in response to the Russian invasion of Ukraine.
But she warned that support for country must not wane.
“In Ukraine, barbarous weapons are being used to kill and maim people," Ms Truss said.
"Rape is being used as an instrument of war. Families are being torn apart.
“And this morning we have seen Putin desperately trying to justify his catastrophic failures. He is doubling down by sending even more reservists to a terrible fate.
“He is desperately trying to claim the mantle of democracy for a regime without human rights or freedoms. And he is making yet more bogus claims and sabre-rattling threats.
“This will not work. The international alliance is strong. Ukraine is strong.”
Ms Truss told fellow world leaders that the UK would spend 3 per cent of its gross domestic product on defence by 2030, repeating a promise she made when she campaigned to become leader of the Conservative Party.
“In the face of rising aggression we have shown we have the power to act and the resolve to see it through. But this must not be a one-off," she said.
“This must be a new era in which we commit to ourselves, our citizens, and this institution that we will do whatever it takes — whatever it takes to deliver for our people and defend our values.”
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Ms Truss praised the “brave, dignified” Ukrainian first lady Olena Zelenska, who is attending the summit in New York.
She also drew an explicit link between the war in Ukraine and the energy crisis facing the UK and countries across Europe, as she promised not to be affected by cutting off “toxic power and pipelines” from authoritarian regimes.
“We will ensure we cannot be coerced or harmed by the reckless actions of rogue actors abroad," Ms Truss said.
“We will transition to a future based on renewable and nuclear energy whilst ensuring that the gas used during that transition is from reliable sources, including our own North Sea production.”
Earlier on Wednesday, Mr Putin's comments in a televised address to the nation appeared to suggest the conflict in Ukraine could become a nuclear crisis.
He announced a partial military mobilisation, with 300,000 reservists set to be called up as the Kremlin tries to regain ground from a counter-attack by Ukraine’s forces.
And Mr Putin said “it’s not a bluff” when he vowed that Russia would use its weapons of mass destruction if its territory were threatened.
He accused the West of “nuclear blackmail” and claimed “high-ranking representatives of the leading Nato states” had talked about the possibility of using weapons of mass destruction against Russia.
“To those who allow themselves such statements regarding Russia, I want to remind you that our country also has various means of destruction,” Mr Putin said.
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His comments prompted stern responses from other world leaders.
US President Joe Biden used his speech at the summit to condemn Mr Putin’s “reckless disregard” for the country’s responsibilities as a signatory to the Treaty on the Non-Proliferation of Nuclear Weapons.
European Commission President Ursula von der Leyen, after a meeting with Ms Truss, said: “We condemn Putin’s actions and agree that his calls to mobilise parts of the population are a sign of weakness. Russia’s invasion is failing.”
The referendums in pro-Moscow regions of Ukraine could give Russia the pretext for a wider war because Mr Putin would be able to claim parts of his state were being attacked.
Foreign Secretary James Cleverly said the UK would never recognise the results of “sham referendums” that were “held at the barrel of a gun”.
Defence Secretary Ben Wallace said Mr Putin’s actions were “an admission that his invasion is failing” and “Russia is becoming a global pariah”.
Melinda Simmons, the UK’s ambassador to Kyiv, said his “essential weakness” was “he still refuses to understand Ukraine”.
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A British defence intelligence update suggested Mr Putin was being forced to undermine his own public position that the war in Ukraine was a “special military operation” rather than a full-scale conflict.
“These new measures have highly likely been brought forward due to public criticism and mark a further development in Russia’s strategy,” the UK Ministry of Defence said.
“Putin is accepting greater political risk by undermining the fiction that Russia is neither in a war nor a national crisis, in the hope of generating more combat power.”
Ms Truss, who held talks with Mr Biden on Wednesday, is using her visit to the UN to rally support for Ukraine.
She joined Ms Zelenska and Ukrainian Prime Minister Denys Shmyhal to tour an exhibition titled Russian War Crimes at the Ukrainian Institute of America on Tuesday evening.