Donald Trump has backtracked on his proposal to ban Muslims from entering the United States, saying it was "only a suggestion".

The presumptive Republican presidential candidate sparked global outrage in December when he called for a "total and complete shutdown" on Muslim travel to the US in the wake of the Paris attacks which left 130 people dead.

A petition to ban the controversial billionaire from the UK attracted more than half a million signatures with politicians even debating it in Parliament.

But Trump, now assured of the Republican nomination, has appeared to soften his rhetoric.

“It’s a temporary ban, it hasn’t been called for yet, nobody’s done it, this is just a suggestion until we find out what’s going on," he told Fox News.

He was responding to criticism from London's newly elected mayor Sadiq Khan.

Sadiq Khan, the new mayor of London. Credit: Reuters

Trump had said Khan, the capital's first Muslim mayor, would be exempt from the ban.

But Mr Khan refused the offer, telling ITV News that Trump's policies would make the US and UK less safe.

It is not the first time Trump has shifted position on key election issues - he has previously changed his views on abortion, tax, healthcare, guns and even his likely presidential opponent, Hillary Clinton.