In a surprise regulatory statement, the company said Mackenzie had been removed by the board from his role as executive chairman, from his other roles and as a director and as an employee of the Company, for gross misconduct, with immediate effect.

There is no suggestion that any fraud or financial issues are behind the dismissal, which was later said to involve a ‘personal conduct related matter’.

Subsequently Mackenzie's son said his father had ‘tendered his resignation this morning... due to acute ill health’, and had been suffering symptoms ‘for some time’.

Peter Mackenzie said: ‘This is an extremely distressing mental health issue.

‘A consultant clinical psychologist advised him last week that he needed to take at least six months leave. He is very unwell and has been admitted to hospital’.

After qualifying as an ACA in 1978, Mackenzie subsequently worked in private equity and has also held senior executive roles with BET, as well as becoming CEO of Sea Containers, and chairman of Northgate and also of PHS Group.

He was previously the chairman and CEO of National Car Parks, and became chairman of AA in 2014 after leading a management buy-in. News of his sudden departure saw AA shares drop by almost 18%.

Mackenzie’s position as chairman has been taken by John Leach, a fellow of ICAEW and turnaround specialist, who joined the AA in June 2014 as a non-executive director and is also chair of the nomination committee and a member of the risk and audit committees. Simon Breakwell, the former of Expedia, has been appointed as acting chief executive.

AA said in a statement that its board had already begun the process of splitting the combined roles of executive chairman ‘as the company nears the end of the transformation programme’. The company said the nomination committee will now accelerate this appointment of a chief executive.