Mr Biden has provided US military aid for the war but could lose power in this November’s presidential election. Trump has instead promised to curtail support and end the war “in one day” through negotiations with Russia.
Mr Colby, who served in the Pentagon in Trump’s first administration, is one of several Trump allies who has held talks with Labour to discuss how the US and UK would work together if the former president wins power for a second time.
“The biggest thing on the agenda with European states should be: are you taking primary responsibility for the conventional defence of Nato in Europe?” he said.
“You guys have got to take care of your own situation better, because we have to focus on China.”
He said Britain should deploy a “Britain first” strategy focused on European security and abandon its “tilt” to the Indo-Pacific region.
“Britain obviously is located in Europe, so its interests are more engaged there, and that’s where there’s a gap in the overall American-led order of military capability,” he said.
“There’s very little the British military can realistically do at this time in the Pacific or further afield.”
‘Labour has restrained view of American power’
Mr Colby said Labour had been more willing than Rishi Sunak’s government to accept Trump’s plan to reduce American involvement in Ukraine.
Lord Cameron, the former foreign secretary, was criticised by Republicans in April after he visited Washington to plead with Congress to agree more military support for Kyiv.
“I’m sure [Labour] will make the advocacy for Ukraine, but the spirit that they give off is one that’s more understanding of a more restrained view of American power and American intervention,” he said.
“That is understanding the American people where they are, as opposed to what I heard from David Cameron and so forth, which was much more like political haranguing.”
Other Nato countries have been sceptical that they could make up the shortfall in military support for Ukraine if Trump withdrew American aid.
‘Americans tired of British demands’
The US is by far the largest contributor to Ukraine’s defence, spending £62.5 billion on the war since 2022, compared with £86.2 billion from all European countries combined.
But Mr Colby said Americans were tired of hearing demands from British politicians on Ukraine, including over the latest funding bill, which is known as a “supplemental”.
“I would much rather have the United States dealing with a practical, hard-headed, realistic, serious, Europe-focused UK from a defence point of view, than one that is intervening in the supplemental debate, pushing us to give Storm Shadow [missiles], and then only giving £3 billion a year,” he said.
“It is basically writing cheques that the American people are supposed to cash.”
Sir Keir has said he will work with any US administration, including one helmed by Trump, although he has suggested he hopes Mr Biden will win November’s election.
In an op-ed for The Telegraph on Monday, Mr Lammy and Mr Healey said Labour will pursue a “Nato-first” defence strategy and “reset” defence relations with Europe.