With the price of bitcoin moving toward $12,000, a top economist on Tuesday sent a stark warning to investors: The cryptocurrency is in a "dangerous speculative bubble."

"This is a toxic concept for investors," said Stephen Roach, Yale University senior fellow and the former Asia chairman and chief economist at investment bank Morgan Stanley.

Roach, described by Yale as one of Wall Street's most influential economists, spent the bulk of his 30-year career at Morgan Stanley heading up a highly regarded team of economists around the world.

He had a critical take on the explosion of buying the world's most popular cryptocurrency.

"This is a dangerous speculative bubble by any shadow or stretch of the imagination," he told CNBC's "The Rundown."

"I've never seen a chart of a security where the price really has a vertical pattern to it. And bitcoin is the most vertical of any pattern I've ever seen in my career," he added.