- Sir Nigel Sheinwald said that Johnson had been a poor foreign secretary and had let down Sir Kim Darroch in his row with Donald Trump.
- Sheinwald's comments came after Johnson was accused by colleagues of throwing Darroch "under a bus."
- Johnson again refuses to criticise Trump, after his latest attack on prime minister Theresa May.
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Boris Johnson is a "fully paid-up member of the Donald Trump fan club" and now risks doing "great harm" to Britain's interests abroad, according to a former UK ambassador to the US.
Sir Nigel Sheinwald, who served in Washington under David Cameron, said that Johnson had been a poor foreign secretary, who risked damaging Britain's interests abroad, if he becomes prime minister later this month.
"If Boris Johnson doesn't adapt his style to being prime minister it will do great harm to British foreign policy," Sheinwald told ABC Australia.
"I don't think he was a good foreign secretary. That's a British understatement. He's going to have to change something in his act to be a credible international player."
Sheinwald said that Johnson's refusal to back his successor, Sir Kim Darroch, in the row over his leaked comments about Donald Trump, had been the "last straw" in forcing Darroch to resign.
He said that Johnson had failed to stand up for Britain's interests, because of his desire to "slavishly" back Trump.
"He's a fully paid up member of the Donald Trump fan club," Sheinwald said.
"I think that's a problem. I think we have to project British interests to the US administration not simply play back into Britain what the United States wants at a time when they' view are not widely supported [in the public]."
Sheinwald predicted that Johnson's attempts to win over Trump would go down badly with the British public.
"The UK is much closer on the substance to the European centre of gravity and not to resident Trump and so if Boris Johnson comes in and tries to shift things much, much further in a pro-American direction that will be very difficult for British public opinion which by and large doesn't want to be slavishly close to the United States," he said.
"We want to have a very good relationship but not at any cost."
Johnson finds 'it hard to disagree' with Trump
Johnson has suffered a major backlash from his party colleagues and political opponents after refusing to say that Darroch should keep his job, after Trump labeled him "whacky," "stupid" and insisted the White House would not deal with him.
Foreign Office minister Sir Alan Duncan on Wednesday accused Johnson of having "basically thrown our top diplomat under a bus."
He said that Johnson's refusal to defend Darroch was "pretty contemptible," adding that: "There are a lot of people here in the Commons who are very, very angry and feel he has lost so much respect for having done what he's done."
Conservative MP Sir Patrick McLoughlin agreed, saying: "It is unedifying to see someone who wants to be Prime Minister failing to stand up for hard working civil servants, who have done nothing wrong, under attack from foreign governments.
"Leadership involves standing up for your team. If we don't call out those who want a witch-hunt through the civil service we are complicit in creating divisions that may never heal."
The opposition Labour party also rounded on Johnson, accusing him of failing to stand up for the UK.
"Johnson is effectively behaving as a Donald Trump patsy," a spokesperson for Labour leader Jeremy Corbyn said on Wednesday.
"He won't stand up to Donald Trump and be won't stand up for Britain."