National Australia Bank announced the resignation of its chief operating officer Antony Cahill yesterday. The bank said Cahill, who has been with NAB for eight years (four years on the executive team) and was regarded by some as a potential successor to CEO Andrew Thorburn, would return to the UK to take up a senior role with a London-based "global payments technology company".NAB later confirmed that Cahill is to take on the role of managing director of Visa's European operations from January. He finishes up at NAB on September 14. Executive general manager deposits & transaction services, Rachel Slade, has been appointed acting group executive customer products & services, while the bank considers "next steps"."Ms Slade's appointment is effective immediately, to enable a smooth transition - subject to any applicable regulatory approvals and registration requirements," NAB said.Cahill was a very public face of the bank and fronted up to parliamentary committees alongside Thorburn. His departure slashes the field of potential successors to Thorburn for the CEO role, with the odds-on favourite now former NSW premier Mike Baird, who is currently chief customer officer for NAB's corporate and institutional bank.