Mackay underwent surgery on Monday (22 April) and would now embark on a course of treatment, the company said.
Clark was appointed chief operating officer in July last year with a view to succeeding Mackay in July this year. Mackay was due to become the company's non-executive chairman from that date. Clark now takes over with immediate effect.
"The board will keep Mackay's position under review pending the outcome of his treatment and John Manser, deputy chairman, will become acting chairman for the time being," the group said.
Manser said Mackay had made an immense contribution to making SABMiller what it was today but the transition, in accordance with succession plans, was 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," he said.
He wished Mackay a speedy recovery.
Mackay said he did not expect his departure to have any effect on the company's operations or strategic direction, which he said would 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," Mackay said.
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