Shares in SABMiller fell this week after the brewer revealed its highly respected, long-serving boss Graham Mackay was stepping down earlier than expected after being diagnosed with a brain tumour.

Mackay, who has been at the helm for more than a decade, was named executive chairman last year. He was due to step down as CEO and become non-executive chairman in July.

But SABMiller revealed that Mackay had undergone surgery on Monday and that his successor chief operating officer Alan Clark would therefore become CEO with immediate effect.

It would keep Mackay’s position under review pending the outcome of his treatment, it said.

Mackay was named the 11th most powerful person in The Grocer’s Power List 2012 after securing the £6.53bn takeover of Foster’s. Since taking the helm in 1999, shares have soared from around 550p to £33.60 today, while turnover has increased from around $4bn to more than $31bn.

“I am inexpressibly proud of this company, what it has achieved and the shape it is in,” Mackay said in a note to staff. “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 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.”

Deputy chairman John Manser added: “Our thoughts and prayers are with Graham and his family. He has made an immense contribution to making SABMiller what it is today.”