Breaking up the biggest banks would be futile, onetime Citigroup (NYSE:C) eminence and former Treasury Secretary Robert Rubin said on Thursday.

It was the latest repudiation of Sandy Weill's change of heart by someone with ties to Weill. The former Citigroup chief executive, who created the modern megabank, last summer started advocating for the separation of large banks' investment and commercial banking activities. Since then several bankers, regulators and industry veterans have weighed in, keeping the debate about the largest banks' size and utility roiling.

Rubin on Thursday delicately disagreed with Weill, who had "personally recruited" him to Citigroup.

"I have a lot of respect for Sandy, but I think that if you followed Sandy's path and you broke up the banks in some fashion or other, as he's describing, the risk isn't going to go away," Rubin said in a CNBC interview Thursday.

"The systemic risk, the 'too big to fail' risk will move from one place to another place," he added. "For example, if you could curtail what the banks can do in terms of trading, it isn't that the trading is going to go away. You have a large global economy that needs those activities but they'll go to other platforms. I think the real question is, are there ways to deal with the risk?"

Former Treasury Secretary and Rubin protégé Timothy Geithner "probably had the best answer" to that question, in advocating for banks to build up their capital levels, Rubin said. Geithner is rejoining the Council on Foreign Relations, where Rubin is a co-chairman.

Rubin has been criticized for his role at Citigroup in the years leading up to the financial crisis; as a director and a senior advisor who collected some $126 million in compensation from the bank, critics say he could have done more to prevent the bank's massive losses on mortgage-backed securities. Citigroup wound up taking $45 billion in bailouts from the U.S. government during Rubin's watch; the Treasury Department only got rid of the last of the Citi securities it acquired in the bailouts this week.

A "Frontline" television program last month focused in part on Rubin's role at Citigroup, noting that an internal whistleblower warned him in 2007 that the company was buying huge amounts of bad mortgages. When a financial crisis inquiry panel asked Rubin in 2010 how he reacted to that email, he told them, "Either I or somebody else sent it to the appropriate people."

When asked on Thursday if he felt any responsibility for what happened to Citigroup, Rubin largely demurred.

"I was worried about excesses before the crisis began. But what I didn't see and virtually nobody saw … was the possibility of a serious crisis. It turned out to be the worst crisis in about 80 years. I wish I had seen it," he said. "I regret not having seen it, and I would suspect or would guess that there are very large numbers, vast numbers of other people who have the same view, that is to say, who also regret not having seen it."

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