Global investing maven Mark Mobius had unforgiving words for the U.K.'s leadership ahead of crucial Brexit votes this week.

Speaking to CNBC at the Global Financial Forum in Dubai, the asset management mogul said it was Prime Minister Theresa May's time to depart.

"I think May has got to go," Mobius, founder of emerging markets fund Mobius Capital Partners, said Monday. "I think she doesn't have the leadership capability. You've got to have somebody who is going to be willing to say what she thinks, personally, and lead and say 'Look, I don't believe in Brexit, and we should change."

"I definitely think she's against Brexit — she was in the beginning. I'm sure now it's even more."

Indeed, the embattled prime minister quietly supported remaining in the European Union before the June 2016 Brexit vote, warning of adverse effects to the U.K.'s economy, security and even its union with Scotland. Since becoming head of government, however, she has pledged to "honor the results of the referendum," reiterating that "Brexit means Brexit."

The U.K. is scheduled to leave the EU on March 29.