Lord Cameron will warn in his visit to Washington DC next week that the United States is risking the West's security by blocking a fresh aid package for Ukraine.
In a face-to-face meeting, the Foreign Secretary will urge House of Representatives Speaker Mike Johnson to bring an end to his Republican colleagues' opposition to the aid.
Lord Cameron and Stéphane Séjourné, his French counterpart, said: 'We are both absolutely clear – Ukraine must win this war.' They warned: 'If Ukraine lose, we all lose.'
Earlier this year, the UK government pledged some £2.5 billion in aid to support Ukraine's fight against the Russian invasion, and the European Union offered a further £43 billion.
However, Republican opponents in the House of Representatives – who make up half of the US Congress – are currently blocking Joe Biden's £75 billion ($95 billion) bill which contains aid for Ukraine.
Lord Cameron will warn in his visit to Washington DC next week that the United States is risking the West's security by blocking a fresh aid package for Ukraine
He will urge House of Representatives Speaker Mike Johnson (pictured) to bring an end to his Republican colleagues' opposition to the aid
Issuing a rallying cry for the extra funding to be approved, Lord Cameron wrote in The Daily Telegraph: 'The costs of failing to support Ukraine now will be far greater than the costs of repelling Putin... we must do even more to ensure we defeat Russia. The world is watching – and will judge us if we fail.'
They added that 'it is not for France and Britain alone to solve these challenges' and urged that 'we can rally others to join us in overcoming them'.
Ukrainian president Volodymyr Zelensky has also warned that Ukraine would lose the war unless Congress passed the military assistance package.
Ukrainian president Volodymyr Zelensky has also warned that Ukraine would lose the war unless Congress passed the military assistance package
Firefighters inspect damaged fire truck after a Russian drone attack on April 4 in Ukraine
Mr Zelensky said: 'It is necessary to specifically tell Congress that if Congress does not help Ukraine, Ukraine will lose the war.
'If Ukraine loses the war, other states will be attacked.'