'Loose Cannon' Yellen as Fed Chief Scares Market: Janjuah

Fed Might Target Rate Guidance to Data, Not Dates: Yellen

Global markets are riding a "high" on central bank money printing, but a change at the top of the Federal Reserve posed the biggest threat to the rally, Bob Janjuah, Nomura's uber-bearish strategist, told CNBC on Friday.

Janjuah said Janet Yellen, Fed vice-chairman since 2010 and a likely successor to Bernanke when he steps down in January 2014, had the potential to scare the bond markets.

"A client said to me a few weeks ago that if Karl Marx was in charge of the world, he'd have Janet Yellen as his central bank governor. She is far looser or more dovish than Bernanke," Janjuah said on CNBC Europe's "Squawk Box."

"My issue with somebody like Yellen is that she's potentially perceived as such a loose cannon - so loose – that it would scare the bond market, whereas Bernanke has built up a certain level of credibility with the market."

(Read More: Central Banks Gone Wild: What Can Investors Do?)

Yellen is seen as one of the most dovish members of the Fed's policy-setting committee. She has supported the continuation of the Fed's unconventional asset purchase strategy to get the economy growing, while other members have called for an end to the purchases.

Janjuah said "QE infinity" had driven markets to new highs but that rally could end when Bernanke leaves.

(Read More: I'm Not Confident About US Markets: Jim O'Neill)

"The message from me is that anyone who is different is a risk for the market," he said. "When we transition from one Fed chairman to another, things tend to happen. When we went from Volcker to Greenspan in October 1987, markets dropped up to 35 percent…so any change in that figure-head and chairman's role is a risk for the market," he added.

"The big one for me isn't so much in the detail of what the Fed might, or might not do in the next few months, it's the fact that Bernanke may well not be in that job come January next year."

Yellen No 'Crazy Dove'

Julia Coronado, a chief economist from BNP Paribas, defended Yellen by telling CNBC's "Worldwide Exchange" that Yellen was not the "crazy dove that sometimes people make her out to be."

"I couldn't disagree with Bob Janjuah more, to call Janet Yellen a 'loose cannon', I don't know which Janet Yellen he knows," Coronado told CNBC on Friday. "If you look at her speeches and track record, she is one of the most measured and experienced people. I think she will take the role when Bernanke steps down next year. She is clearly the most qualified and markets would have a great deal of confidence."

"She's a very reasonable person, she's a very good economist and she's been there every step of the way through these unconventional policies, that's critical for markets," she added.

Patrick O'Keefe, director of economic research at CohnReznick agreed saying that he thought Yellen was "more than capable of taking over the chair of the Fed and continuing the policy they've embarked upon."

"Whether that policy is a wise policy is the real question, but she would be more than able to change course when the Fed sees data that justifies that," he added.

The Federal Reserve has said that it could start tapering its quantitative easing program when the labor market improves, specifically, when unemployment drops to 6.5 percent.

-By CNBC's Holly Ellyatt, follow her on Twitter @HollyEllyatt