Co-operative financial Services' (CFS) chief executive has quit in the middle of the group's attempt to pull off the transformational acquisition of a branch network from Lloyds Banking Group.
Neville Richardson has stepped down immediately as the boss of CFS and deputy chief executive of the Co-operative group.
His departure comes almost two years after CFS merged with Britannia Building Society, where Mr Richardson was chief executive.
Barry Tootel, the finance director of CFS, will take over as acting chief executive of the business. The Co-op said Mr Richardson would work with Mr Tootel during a transition period but did not give more details.
The group also declined to comment on whether Mr Richardson would receive any form of pay-off.
His departure is a surprise because the customer-owned Co-op is one of few serious bidders for 632 branches that Lloyds is selling, with offers due by the end of this month.
The deal would be a massive expansion for CFS's banking operation, which has 350 branches, and would probably be the group's biggest acquisition.
Mr Richardson said he always intended to look for "new challenges" when the merger of CFS and Britannia was well advanced.
"That time has now come and I want to move on to the next stage of my career," he added.
Only last week, Mr Richardson ordered the closure of CFS's door-to-door sales operation with the loss of 670 jobs – a move that aroused anger among unions.
CFS weathered the financial crisis well and increased operating profit by 18 per cent to £209m last year. Its ethical policies, such as not investing in animal experiments and arms companies, helped drive a 79 per cent increase in current account switchers.
Peter Marks, the Co-op's group chief executive, said Mr Richardson had beaten pre-merger targets and achieved record financial results as head of CFS.
"We are grateful to Neville for all he has done at CFS and at Britannia in the preceding years and wish him every success," he added.