Tim Tookey, Lloyds' finance director, is to resign to become chief financial officer at Friends Life.
He was a pivotal figure in helping Lloyds to secure a crucial £20bn-plus recapitalisation two years ago, the Financial Times reports.
Tookey denied the move was due to any falling out with the bank's new chief executive, António Horta-Osório, who joined the bank from Santander UK and took over as CEO in February.
He was believed to have concerns about the credibility of Lloyds' new financial targets, especially given the discouraging macroeconomic backdrop.
However, he denied there had been any falling out with Horta-Osório or any disagreement over Lloyd's strategic direction: "We have spent a lot of time together and I have mixed feelings about moving on, but the time is right for a new challenge."
No replacement has been lined up and an external search will now begin, according to the FT.
At Friends, Tookey will link up again with Andy Briggs, who was a senior insurance executive in Lloyds' life and general insurance businesses until early this year before becoming chief executive at the life company. The two men's paths also crossed during their time at the UK arm of Prudential.
Tookey, who has been at Lloyds for six years, is expected to remain in his job until the end of February, for the presentation of 2011 results.
Lloyds declined to comment.
Smoking biggest culprit; obesity second
Average earner will gain £840 in 2018
Will also move heritage items
Responding to letter from Treasury Committee chair Nicky Morgan