Stagecoach co-founder and billionaire businessman Sir Brian Souter will leave his position as chairman from the beginning of next year.

He will stay on the firm's board, but his position as chairman will be taken by Ray O'Toole.

Brother-and-sister team Sir Brian and Dame Ann Gloag, who will retire from the board completely, set up Stagecoach.

The siblings helped grow the business into what is now claimed to be Britain's biggest bus, coach and tram operator, employing 24,000 people across the country and with a fleet of more than 8,300 vehicles.

Sir Brian said: "At the age of 65, the time is right for me to step down as Stagecoach chairman to spend time on my other interests and with my family, including my three young grandchildren.

"My family and I continue to have a significant shareholding in Stagecoach and I have every confidence in the management team, our strategy and the positive prospects of the business.

"I look forward to continuing to represent the interests of stakeholders as a non-executive director on the doard."

READ MORE: Scottish business chief hails ‘excellent’ investment showing

Dame Ann Gloag and Sir Ewan Brown, both long-serving non-executive directors, will retire from the board on 31 December 2019.

Mr O'Toole said: "The contributions of Sir Brian Souter and Dame Ann Gloag both to the public transport industry and to Stagecoach as co-founders, senior executives and directors cannot be overstated.

"The insight and wisdom of Sir Ewan Brown over his many years as a director of the company have also been key in its success.

"On behalf of the company, I extend my sincere thanks to all three of them and I look forward to continuing to work with Sir Brian and having access to his insight on the board.

"I am delighted to have the privilege of succeeding Sir Brian as Chairman of Stagecoach, a company which continues to innovate and lead the sector, and deliver consistently for its customers.

"Most of my career has been in public transport and I am confident about the prospects for sustainable mass transit and for Stagecoach."

READ MORE: Sir Brian defends Stagecoach rail record as investors revolt on pay

Revenue plunged at bus group Stagecoach in the first half of the financial year, as the company continued to scale back the size of its business.

It hit £800 million in the first six months, down from just over £1 billion in the same period last year.

Profit before tax rose 35% to £65.9 million. On an adjusted basis, pre-tax profit fell 8.9% to £66.6 million.

"Our updated strategy is based on three key objectives: maximise our core business potential, manage change through our people and technology, and grow by diversifying," said chief executive Martin Griffiths.