The departure of Marks & Spencer finance supremo Alan Stewart looks as if it came as unpleasant surprise to the food and fashion giant.

News that Stewart is jumping ship to Tesco, where he will succeed Laurie McIlwee as chief financial officer, came less than a fortnight after he took on additional executive responsibilities.

At the end of June Stewart added property to his list of executive duties. It was part of a wider rejig of roles among M&S’s top brass and it would be surprising if that had happened if the board knew he would shortly be on his way.

Stewart’s assumption of property responsibilities was emblematic of the central role he has played at M&S chief executive Marc Bolland’s side since his appointment in October 2010.

He has been deeply involved in projects such as the overhaul of M&S’s systems and distribution network, marrying a sharp financial view with a strategic outlook in the ongoing efforts to modernise the business and restore its success.

Stewart frequently fielded a wide variety of questions from the press at results and strategy presentations, indicative of the central role he was playing. On occasion he would go further than Bolland in explaining decisions or rebutting the criticisms voiced by the media pack.

He leaves M&S at a time the business remains in the midst of change and faces ongoing challenges to improve its performance in clothing and address problems with its new website.

However, while some might question his decision to leave when the task remains uncompleted, the difficulties of M&S have not diminished his reputation as a top finance director.

He won wide recognition for his money skills at WHSmith where, along with Kate Swann, he ensured the bookseller and stationer continued to deliver what many thought impossible – continually rising profits despite falling sales.

And although he leaves M&S not yet firing on all cylinders, Stewart is clearly not put off by a challenge. Tesco, to similar degree to M&S, is mired in trouble – trouble that led to sustained sniping against his predecessor McIlwee.

Tesco will pay Stewart a basic salary of £750,000 and replacement shares valued at £1.7m in lieu of deferred awards from M&S.

Tesco chief executive Phil Clarke said: “When we set out on this search we wanted a candidate who had the right blend of experience, leadership and values to play a leading role in the transformation of Tesco. We have found all three in Alan.”

The question being asked in the retail industry is to what extent Clarke’s gain will be Bolland’s loss as M&S confronts another upset in its drive to return to form. A search for Stewart’s successor at M&S has begun.