Rob Burgess, the £250,000-a-year head of Alliance Trust Savings and a board member of Alliance Trust, has stepped down with immediate effect.

There was no explanation from Alliance, and no indication from Mr Burgess that he is leaving to pursue other opportunities.

But a close observer of the business said there had been "a unanimous board decision" to instigate a search for someone who could lead ATS into the next stage of its growth.

During the campaign against Alliance by activist shareholder Laxey Partners last year, the ongoing losses run up by ATS, along with Mr Burgess's £20,000 salary rise to £250,000, were seized upon by Laxey.

Alliance countered that ATS was a long-term investment – it has more than 50,000 customers who hold £5.5 billion of holdings including 22% of the Alliance Trust stock.

The competitive retail savings market, however, pits ATS against the likes of Fidelity and Cofunds at a time of new opportunities created by the retail distribution review.

Mr Burgess joined Alliance in February 2009, six months after Katherine Garrett-Cox took over as chief executive.

He had previously worked as sales and distribution director at Thomas Cook and had been a senior executive at Lloyds TSB and Scottish Widows.

Lesley Knox, the outgoing chairwoman of Alliance Trust, said Mr Burgess had "instigated a major repositioning of the business resulting in good growth".