Lord David Cameron is expected to warn the US is risking the security of the West by holding up a new package of aid for Ukraine.
He will make the point when he visits Washington DC next week and has a meeting with Mike Johnson, the House of Representatives speaker, to stop his fellow Republicans from blocking the funding.
The UK has already pledged £2.5 billion in aid earlier this year to help Kyiv take on Russian forces and the EU has also vowed to give £43 billion.
The £75 billion Bill put forward by US President Joe Biden, including security assistance to Ukraine, is currently being blocked by the Republican side of US Congress.
Lord Cameron and Stéphane Séjourné, his French counterpart, in a joint Telegraph article to mark Monday’s 120th anniversary of the Entente Cordiale alliance between the two nations.
They wrote: “We are both absolutely clear – Ukraine must win this war. If Ukraine loses, we all lose.
“The costs of failing to support Ukraine now will be far greater than the costs of repelling Putin. But, as discussed during the Paris Conference in February, we must do even more to ensure we defeat Russia. The world is watching – and will judge us if we fail.”
They added “it is not for France and Britain alone to solve these challenges” and “we can rally others to join us in overcoming them”.
Ukrainian President Volodymyr Zelensky warned on Sunday that his country will lose the war if further US funding is not approved.
In his previous visit to Washington, Lord Cameron likened those blocking US aid to Ukraine to people who appeased Nazi Germany in the 1930s.
His remarks were condemned by pro-Trump Republican Congress member Marjorie Taylor Greene, who told him to “kiss my a**”.
The Foreign Secretary has recently urged all Nato allies to increase their defence spending to 2% of gross domestic product (GDP), to prepare for the possibility of Donald Trump’s return to the White House.
Mr Trump, an outspoken critic of Nato members who have not met their aid commitments, has also expressed scepticism about the US providing aid to Ukraine.
Lord Cameron is also expected to meet Antony Blinken, the US secretary of state, and the pair are likely to discuss the conflict in Gaza.
The UK and the US have hardened their approach towards Israel in recent days after the killing of seven aid workers, including three Britons, by the Israel Defence Forces on April 1.
Lord Cameron on Sunday warned that the UK’s support for Israel was “not unconditional” in response to the air strikes, and previously called for a full and transparent investigation to follow the IDF’s initial report into the incident.