Tory MP Dominic Grieve believes Boris Johnson is "unfit" to be Prime Minister.
The former Foreign Secretary is the favourite to replace Theresa May because he is widely seen as the candidate that gives the Conservatives the best chance of winning a general election.
But Mr Grieve, MP for Beaconsfield, told LBC's Tom Swarbrick that he would consider his position in the party if Mr Johnson was in charge.
"I personally have serious doubts about Boris Johnson's capacity to be Prime Minister," he said.
Asked if he felt he was unfit for the job, he said: "Yes I do. I'm afraid he's shown, especially during his period as Foreign Secretary, that he doesn't have the necessary skills and capacity."
But would he leave the party if the former Mayor of London became leader?
"I would have to consider my position very carefully. I would find it very difficult to accept his leadership," Mr Grieve said.