The Financial Ombudsman Services former chief executive, Natalie Ceeney, has jumped ship and joined HSBC as its head of customer services.
After announcing her resignation from Fos in November 2013, Ceeney left the watchdog after dealing with its one millionth payment protection insurance (PPI) complaint.
Ceeney was seen as the consumer's champion after her attacks on the banks' handling of PPI complaints.
"Disappointingly we are still seeing cases where businesses are not following our long-standing approach to PPI, resulting in long waits and unnecessary delays for consumers," said Ceeney in September last year.
Tony Boorman, deputy chief ombudsman, has taken over from Ceeney since her departure.
"Having just received our millionth new PPI complaint, Ceeney feels that now is the time for her to move on – as the ombudsman service itself starts out on a new set of challenges, building on the foundations for change laid under Ceeney's leadership," said Nicholas Montagu, chairman of Fos.
HSBC said it did not approach Ceeney until after she resigned from Fos but she is the latest regulator to have moved from battling the banks to joining them.
The Financial Conduct Authority's acting director of retail Christina Sinclair quit the watchdog to join Barclays.
Like Ceeney, Sinclair played a leading role in campaigns to provide redress for the mis-selling of payment protection insurance (PPI) and interest rate hedging products but after 18 years swapped sides.