SABMiller, the international brewer, has accelerated the promotion of chief operating officer Alan Clark to chief executive with immediate effect following news that executive chairman Graham Mackay has been diagnosed with a brain tumour.

In a statement released today, the company said Mackay underwent surgery yesterday (22 April) and will now embark on a course of treatment.

Clark was appointed chief operating officer in July 2012 with a view to succeeding Mackay as CEO in July 2013 after a transitional period of one year, when it was intended that Mackay would become non-executive chairman.

SABMiller said: “The board will keep Mr Mackay’s position under review pending the outcome of his treatment, and John Manser, deputy chairman, will become acting chairman for the time being.”

Manser said: “Our thoughts and prayers are obviously with Graham and his family at this difficult time. We wish him well and look forward to his recovery.

“He has made an immense contribution to making SABMiller what it is today, but the transition from Graham to Alan in accordance with our succession plans is already in its final stages. SABMiller has a strong management team, with considerable depth of talent, which I have seen for myself during my 12 years on the board, and we are confident that under Alan Clark’s leadership the management team will continue to grow our business and deliver value to shareholders.”

In a note to SABMiller employees, Mackay said: “I am inexpressibly proud of this company, what it has achieved and the shape it is in. I certainly do not expect there to be any impact from this development on its operations or strategic direction, which will continue to run as normal.

“The transition of management responsibilities to Alan Clark is already well advanced. The group is in excellent hands, and Alan will now simply pick up the remaining executive responsibilities a little sooner than expected. I obviously have an additional personal objective, and that is to get well as quickly as possible for the sake of my wife and children. Thank you all for your support.”