Donald Trump will fail to “make America great again” if he lets Ukraine fall into the hands of Vladimir Putin, Boris Johnson has warned.

The former prime minister urged Republicans in Congress, who have stalled a $60 billion aid package for Kyiv, to unite against Russia to maintain America’s status as the world’s pre-eminent superpower.

Invoking Ronald Reagan, he said the move would deter China from attacking Taiwan, while issuing a defence of Nato and hitting out at Western countries for appeasing Vladimir Putin.

Mr Johnson endorsed Mr Trump’s run for the presidency in January at the start of the primary season, saying he did not believe the Republican would “abandon” Ukraine.

However, his comments on Wednesday – at a conservative conference in Canada alongside former Australian prime minister Tony Abbott – suggest that belief has wavered. 

It follows reports that Mr Trump has drawn up a plan to let Moscow seize parts of eastern Ukraine and maintain its grip on Crimea.

“If you are the party of Ronald Reagan, if you want to make America great again, then you don’t begin a new Republican presidency – which may well happen… by conceding victory to Vladimir Putin,” Mr Johnson said.

“It would be a disaster for the West and it would be a disaster for America… Ukrainians are a free sovereign, independent, European nation and what has happened to them is criminal and barbaric”.

Mr Johnson endorsed Mr Trump in a Daily Mail column this year, noting that he had supplied Javelin anti-tank weapons that had proved critical in the Battle for Kyiv in the early days of the war.

“I ­cannot believe that he will want to go down in history as the president who abandoned a country that he has already signally helped to keep free”, he wrote in January.

However, the Washington Post reported last week that Mr Trump would pressure Ukraine to make peace with Russia by ceding Crimea and the Donbas border region if he wins the presidential election in November.

Hungarian prime minister Viktor Orban said last month that Mr Trump would not “give a penny” to Ukraine, following a meeting with the former US president.

At the same time, Joe Biden’s attempts to keep military aid flowing to Kyiv have been blocked by Republicans in the House of Representatives.

Mike Johnson, the House Speaker, has so far refused to introduce a vote on the package – but has indicated he will do so this week. The move could lead to an attempt by supporters of Mr Trump to unseat him.

Mr Johnson said he believed the House Speaker was a supporter of Ukraine and argued that sending military aid was in the US’ own national interest.

“If you’re worried about the long-term ability of America to remain the superpower, if you’re worried about democracy around the world, if you’re worried about security, then investing in the security of Ukraine is the most cost-efficient thing you could possibly do,” he said.

“I would say, Mike – there aren’t any American boots on the ground in Ukraine, and yet those Ukrainians are fighting for freedom and security everywhere”.

Lord Cameron, the Foreign Secretary, was snubbed when he attempted to meet the House Speaker to discuss the Ukraine package on a visit to Washington this week.

He did, however, meet with Mr Trump at his Mar-a-Lago resort, where he reportedly urged the former president to help push the aid package through Congress.

Meanwhile, Mr Johnson said an Israeli airstrike on a convoy of aid workers last week was “appalling” but criticised those calling on Britain to suspend arms sales to Israel.

“Logically, what we seem to be saying by that action is that we’re willing to see Hamas win,” he claimed.

The move would allow Hamas to repeat its Oct 7 massacre “again and again”, Mr Johnson said, adding that there was a “clear moral difference” between the actions of the terrorist group compared to Israel.

“Israel and the IDF have done their best to minimise civilian casualties, minimise suffering, and Hamas as far as I can see are trying to maximise suffering,” he said.