The fate of Ukraine remains “in the balance”, foreign secretary Liz Truss has warned as she called for allies to increase defence spending and supply tanks and warplanes to Kyiv.

       In a major speech, Ms Truss will argue that Vladimir Putin’s invasion of Ukraine shows the need for a shake-up of the international structures which failed to prevent Russia’s actions.

       She will argue that Western allies should impose tougher economic sanctions, including cutting off oil and gas imports “once and for all”, adding: “There must be nowhere for Putin to go to fund this appalling war.”

       Ms Truss will use her speech at London’s Mansion House on Wednesday to say: "We cannot be complacent – the fate of Ukraine remains in the balance.

       “And let’s be clear – if Putin succeeds there will be untold further misery across Europe and terrible consequences across the globe. We would never feel safe again.”

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       Mr Putin’s foreign minister, Sergei Lavrov, has warned that weapons supplied by Western countries “will be a legitimate target” and Nato has effectively “entered into a war with Russia through proxies”.

       But the UK has backed Ukrainian strikes on military targets behind Russian lines – even if the weapons used have been supplied by the UK.

       Armed Forces minister James Heappey said “of course” the UK backs reported Ukrainian strikes on fuel depots in Russia, saying it was “completely legitimate”.

       Ms Truss will say: “We must be prepared for the long haul and double down on our support for Ukraine. Heavy weapons, tanks, aeroplanes – digging deep into our inventories, ramping up production. We need to do all of this.”

       The UK was one of eight Nato members to meet the alliance’s goal of spending at least 2 per cent of gross domestic product (GDP) on defence in 2021.

       Ms Truss will argue that there has been a “generation of under-investment” in the west and the 2 per cent target should be “a floor, not a ceiling”.

       The foreign secretary is also expected to say that the Russian invasion shows “the architecture” of the international order has failed Ukraine.

       Russia’s position as a veto-wielding member of the UN Security Council has limited that body’s ability to censure the Putin regime. Ms Truss will set out a new approach which recognises “we’re seeing the return of geopolitics”.

       Ms Truss wants to see the G7, the UK, US, Canada, Japan, France, Germany and Italy, playing a stronger role on the world stage. She also wants to forge a stronger network of bilateral security and economic partnerships.

       An ally of the cabinet minister said her speech is in recognition of the fact that the West has been “asleep at the wheel since the Cold War”.

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       Earlier on Tuesday, Ms Truss condemned the “appalling reports” on the use of rape by Russian forces. She told MPs that the UK was working to collect evidence to “make sure the perpetrators are brought to justice”.

       Ms Truss said the UK wants new international agreement to make the use of sexual violence as a weapon of war a “red line”. She told the Commons: “It needs to be regarded in the same way as the use of chemical weapons.”