The company is a major player

Chief executive Keith Cochrane said: "Ann has made a unique contribution to the development of the group over the last 20 years but is now seeking an opportunity to pursue her charitable and other interests."

Mrs Gloag launched Stagecoach with her brother Brian Souter, the present chairman, 30 years ago with �25,000 of their father's redundancy money.

The group has since become one of the largest bus companies in the world.

The family was hit by tragedy in September last year when Mrs Gloag's son Jonathan died at the age of 28.

Finance director

He had stood to inherit much of the Stagecoach group.

Mrs Gloag's departure will be seen as another blow to the beleaguered company, which earlier this year saw the departure of Mike Kinski, then chief executive, and Larry King, head of the company's American subsidiary Coach USA.

Mr Kinski is now heading the Nomura takeover of Welsh utilities company Hyder.

He was replaced as chief executive by Mr Cochrane, who had been finance director of the group.

The vacancy as finance director was also filled on Tuesday as the company announced Martin Griffiths, currently the group's business development manager, would become the finance director with immediate effect.

Stagecoach issued a profits warning earlier this month and its share price has fallen from a high of 188�p this year to 64�p.