“If it had been any other foreign minister, probably no one would have even noticed he was going.” Crispin Blunt, the Conservative chair of the foreign affairs committee, put his finger on the risks of having Boris Johnson, former mayor of London and the colourful figurehead of the Brexit campaign, as Britain’s chief diplomat.

Johnson was mocked by Moscow at the weekend for cancelling a planned visit, and was also derided by the Liberal Democrat leader, Tim Farron, as a “poodle” – the insult repeatedly directed at Tony Blair over his relationship with George W Bush.

The past week’s momentous events have provided the first hints of what the Trump administration may mean for global politics; but on the domestic stage they are also the toughest test of Theresa May’s bold decision to put the man once regarded as her fiercest rival for the Tory leadership into one of the great offices of state.

Relations between No 10 and Johnson have thawed in recent months, after a series of run-ins during his early months in the job, when outspoken remarks were disowned by May’s team. She even joked publicly at Johnson’s expense, in what appeared to be a deliberate strategy of containing her erstwhile rival.

May’s Lancaster House speech in January setting out her approach to Brexit bore signs of lobbying from the pro-Brexit Johnson camp, and was regarded by his allies as a victory over the more cautious Philip Hammond, the chancellor, who would have liked to keep open the option of remaining in the single market and the customs union.

But away from the cabinet table, Johnson, whose journalistic output was littered with outrageous statements and off-colour comparisons, has still to prove he can be taken seriously on the global stage.

Those who have worked closely with the foreign secretary in the past say his ego is more fragile than it can appear, and he is sensitive to the accusation of being a political dilettante. So his friends reacted angrily to the charge that he had called off his Moscow trip at US secretary of state Rex Tillerson’s behest, describing Farron’s poodle jibe as an “utter disgrace”.

Blunt, a strong backer of the foreign secretary, said Johnson had simply made a pragmatic decision. “You’re faced with the choice of going to Moscow as a warmup act, or going to try and broker a unanimous position among the G7 – there’s simply no other sensible thing to do,” he said.

The government is still working to understand the meaning of the abrupt change of heart in Washington, which saw Barack Obama’s cautious approach – and Trump’s antipathy to Middle East intervention – pushed aside after the Syrian chemical weapons attack. “What we have seen in the past week is a massive shift in American foreign policy and direction of travel,” said one Whitehall source.

While May’s hand clamped in Trump’s is the most memorable image of the Conservative government’s determination to establish friendly relations with the new team in Washington, Johnson has struck up a strong relationship with Tillerson in recent weeks.

Starting on Monday in Lucca, he hopes to act as the political bridge between a newly assertive White House and the other G7 states, in particular France, Germany and Italy, which are deeply wary of Trump and sceptical about gung-ho intervention in the Middle East. He believes he has pushed Syria up the agenda in Washington, and hopes Britain’s sanctions plan could eventually pave the way to a diplomatic solution.

But the great European powers are unlikely to take kindly to moral exhortation from the man best known as the floppy-haired frontman of Brexit.

For any British foreign secretary, seeking to act as a diplomatic bridge across the Atlantic has always been a risky exercise. For Johnson, the stakes – both political and personal – could barely be higher.