Article:21 July 2016 Last updated at 20:58 BST
Speaking in an interview with BBC Newsnight's Evan Davis, he said it was "a job that doesn't exist" which would require a "constitutional change" - and he compared the role to that of "director of football".
Watch the full interview on BBC Newsnight on 21 July at 22:30 BST on BBC Two.

Summarize the main points of that article.
Labour leader Jeremy Corbyn has dismissed an offer from Owen Smith of becoming party president in the event his rival wins the Labour leadership.