A prominent U.K. economist and former chairman of the country's financial regulator has split with his contemporaries by declaring that immigration was a "legitimate concern" in the Brexit vote last year, which he believes is now being overlooked in the current negotiations with the EU.

"I think that the most fundamental reason why the Brexit referendum passed was immigration. And I think that was a legitimate concern and in that sense I break away from many of my metropolitan, liberal friends in believing that there was a real issue there," Adair Turner, former Financial Services Authority chairman, told CNBC on the sidelines of the Ambrosetti Forum in Italy.

He added that the disappointing progress in Brexit talks was due to a lack of focus on this particular issue. "I don't think they're going all that well," Turner said about Brexit negotiations. "I think the trouble is that the negotiating stance of the U.K. is muddled; it doesn't simply focus on saying look we've got to get a greater degree of control, certainly of short-term immigration surges, we've got to be able to control immigration flows better and that's all that really matters."