Two of Wall Street's harshest bitcoin critics say investors should remain wary of the cryptocurrency.

"I don't want to be a bitcoin spokesman," J.P. Morgan Chase CEO Jamie Dimon said in a rare joint interview with Warren Buffett that aired Thursday on CNBC's "Squawk Box."

"Just beware,“ he said.

Dimon called bitcoin a "fraud" in September and later said those "stupid enough to buy it" will one day pay the price. He said in January he regretted the "fraud" comment.

In May, Berkshire Hathaway Chairman and CEO Warren Buffett told shareholders that bitcoin is "probably rat poison squared."

"I set a high standard. I don't know whether Jamie can top me or not," Buffett told CNBC during the rare joint interview with Dimon. The interview was recorded Wednesday.

Bitcoin has lost more than half its value since topping $19,000 in mid-December.