The UK must be prepared to go "further and faster" to push Russia out of Ukraine, the foreign secretary has said, in a wide-ranging speech in which she also warned China to "play by the rules" and backed the expansion of Nato.
Liz Truss said on Wednesday night that if Vladimir Putin succeeds in his attempts to seize control of some areas there will be "untold further misery across Europe and terrible consequences across the globe".
According to the Times, the foreign secretary also fears the war could last for at least five years – and even as long as a decade – if Russia establishes any kind of foothold inside Ukraine.
Describing the Russian president as a "desperate rogue operator", Truss insisted that Britain and its allies must keep going "further and faster" to push Russia out of Ukraine.
Putin ordered his troops to invade on 24 February, but despite attempts to take Kyiv Russian troops have been pushed out of the north.
Moscow since launched a concentrated offensive in the Donbas and Crimea, which Putin claims is being carried out to "guarantee the safety" of people in those regions.
In a message that she believes the war in Ukraine could continue for years, she warned: "we must be prepared for the long haul".
Truss said the UK needed to strengthen its military while building alliances with free nations around the world, using their economic power to deter aggressors who “do not play by the rules”.
She said the G7 group of leading industrialised nations should act as an “economic Nato” defending collective prosperity, while the Western military alliance must be prepared to open its doors to countries such as Finland and Sweden.
Speaking at the Mansion House in the City of London, Truss said the international architecture intended to guarantee peace and prosperity had failed Ukraine in the face of an attack by Putin, with no interest in international norms.
“Russia is able to block any effective action in the UN Security Council. Putin sees his veto as a green light to barbarism,” she said.
In the short term, she said Western allies must “double down” on support for the government in Kyiv, providing the heavy weaponry it needs “to push Russia out of the whole of Ukraine”.
At the same time, she said the events of the past months must be “a catalyst for wider change”.
Her speech came as Putin warned any countries trying to interfere in the conflict would face "quick" retaliation.
"If someone intends to intervene in the ongoing events from the outside, and create strategic threats for Russia that are unacceptable to us, they should know that our retaliatory strikes will be lightning-fast," Putin told Russian lawmakers on Wednesday.
"We have all the tools for this, things no one else can boast of having now. And we will not boast, we will use them if necessary. And I want everyone to know that."
He also said attempts by the West to "strangle us economically have failed".
The tensions between Russia and Britain have ratcheted up in recent days, with the Kremlin threatening on Wednesday to strike British military targets in retaliation for the UK's support of Ukraine.
Russian Foreign Ministry spokeswoman Maria Zakharova suggested that Moscow could target territory of Nato countries which have been supplying arms to Kyiv.
Britain has been an outspoken ally of Ukraine, and has supplied weapons including anti-tank weapons and Starstreak anti-aircraft missiles.
When asked of the claims of how long the war could stretch on for, A Number 10 spokesperson said: “I wouldn’t seek to put a timeline on it. We do think that the war has moved into a different phase. This focus on the Donbas region could go on for some time.
"We want it to be resolved as quickly as possible. That’s why we will continue to pressure… on sanctions or providing the sort of military aid to Ukraine that we’re doing on a daily basis.
"But it wouldn’t be right to sort of predict a particular timescale, but it is absolutely right, as the foreign secretary said, that the public should be aware that this could go on for some time.”
Watch: Allies must 'double down' with heavy weapons and air power
In Europe, Truss said, Finland and Sweden should be integrated into the Nato alliance if they want to “as soon as possible”, while states like Moldova and Georgia – which are not Nato members – should have the means to maintain their sovereignty and freedom.
Truss said they had to be prepared to stand up to “aggressors” who try to exploit their economic power as a “tool of foreign policy” to exert control and to coerce others.
“Access to the global economy must depend on playing by the rules. There can be no more free passes,” she said. "We are showing this with the Russia-Ukraine conflict – Russia’s pass has been rescinded.
“The G7 should act as an economic Nato, collectively defending our prosperity. If the economy of a partner is being targeted by an aggressive regime we should act to support them. All for one and one for all.”