Manser was acting chairman for five months earlier this year, when Mackay took medical leave of absence to focus on his treatment following surgery for a brain tumour in April 2013.

Mackay recovered sufficiently to return to his role as non-executive chairman in September 2013, but following a recent sudden worsening of his condition, the board has agreed to his request for a further leave of absence.

The board will continue to keep the position under review.