The global order was more stable and established under Barack Obama's leadership than with Donald Trump at the helm, Republican Senator John McCain has said.

The Arizona senator, a prominent Republican voice on foreign policy, said that Trump is sending a worrying message to other global leaders about America's authority in the world.

McCain believes people around the world "are not sure of American leadership, whether it be in Siberia or whether it be in Antarctica."

The US president's criticism of London mayor Sadiq Khan after the terror attack revealed Trump's unwillingness to step up and act as a leader, according to McCain.

"The message is that America doesn't want to lead," he told The Guardian.

Trump initially criticised Khan for telling Londoners there was "no reason to be alarmed" by the increased police presence in the capital after the terror attack on 3 June which left eight dead and injured 48.

The US president took the London mayor's comment out of context and tweeted: "At least seven dead and 48 wounded in terror attack and mayor of London says there is "no reason to be alarmed!"

When Khan pointed out that he had been misquoted, Trump said the mayor was offering a "pathetic excuse" for his earlier comments.

After the public row between the two politicians, it emerged that Trump has postponed his state visit to the UK over fears that he could face mass protests.

When asked if America's position on the global stage was more secure when Obama was president, McCain said: "As far as American leadership is concerned, yes."

This comes as a surprising admission as McCain has always been a fervent critic of Obama's foreign policy. Last year, he said that Obama deserved an 'F grade' for his unwillingness to lead from the front on the global stage.

"[He] came to power believing that the United States should 'lead from behind.' He believed that we had to get out of conflicts," McCain told US political website The Hill.

"Well, we can't lead from behind," he said, "and he should have learned a long time ago that when the United States gets out of conflicts, that doesn't mean that conflicts end. [The] classic example, of course, is Iraq."

Donald Trump and Sadiq Khan
US President Donald Trump criticised London Mayor Sadiq Khan for his response to the London Bridge terror attack REUTERS/Jonathan Ernst L) and REUTERS/Clodagh Kilcoyne (R/File Photos