"I would not be able to travel to the US under a Trump administration because of his Muslim faith," said Sadiq Kahn in view of his proposed ban on Muslims from travelling to the US.
However, Trump had offered to make an "exception" for London's mayor which has been refused by Sadiq Khan.
Sadiq Khan on Tuesday (May 10) hit out at Donald Trump for signalling that he would exempt him from his proposed temporary ban on Muslims entering the US, saying his "ignorant" comments play "into the hands of extremists".
Donald Trump drew criticism after he proposed a ban on Muslims entering the US after Paris attacks that left 130 people dead in November 2015.
"It's a temporary ban. It hasn't been called for yet," Mr Trump said on Wednesday. "This is just a suggestion until we find out what's going on."
This is not the first time Donald Trump has changed his views on a variety of issues.