Barclays’ chief executive, under investigation for a 2016 incident, has done “a fantastic job” in restructuring the bank amid challenges such as Brexit, according to the British bank’s chairman.

The bank’s CEO, Jes Staley, is being investigated by regulators regarding his individual conduct after he attempted to identify a whistle blower. Staley admitted to making a mistake but was still re-appointed to head Barclays.

He oversaw a large part of the bank’s restructuring process, including selling non-core divisions such as its African business while focusing on retail banking in its home country and corporate and investment units overseas.

“The restructuring of Barclays is one of the most difficult jobs in the world. Remember, with RBS in 2007 and 2008, Barclays was the largest bank in the world by assets and of course that was way overdone. We ballooned the balance sheet and we have to restructure that,” Barclays Chairman John McFarlane told CNBC at the sidelines of the Singapore Summit.

“That restructuring has been very difficult. At this point, we’ve halved the size of the organization and that’s largely been done in the period that Jes has been the CEO,” he said, adding that Staley has done “a fantastic job” and has his confidence to lead the bank.

The chairman had said that he was disappointed with Staley’s conduct in the 2016 incident, but acknowledged that the CEO has been very resilient especially in the face of challenges such as a global structural reform in the banking sector and the U.K.’s impending exit from the European Union.

The bank has indicated that it will move some of its operations to Ireland post-Brexit, but it still hopes to retain most of its activities in London. McFarlane said around 100 jobs could be moved out of the British capital, but “the truth of the matter is we don’t know.”

“The EU has decided that the process is ‘let’s agree on the exit first and the terms of exit, and then we’ll talk about the future relationship.’ From a business standpoint, that’s back to front. If we’re going to talk about an investment in the EU, you want to know what the benefits are at the same time,” he said.

“And so this is causing the maximum amount of uncertainty, particularly for London as a financial center because we don’t know what that relationship is going to be.”

— CNBC’s Sam Meredith and Karen Gilchrist contributed to this report.