A leaked email to Michael Gove from his wife reveals her worries about whether Boris Johnson has enough support to be Conservative leader.
The email, obtained by Sky News, shows Sarah Vine urging her husband to get undisclosed assurances from Mr Johnson before offering any support for a leadership bid.
It said: "One simple message: You MUST have SPECIFIC from Boris OTHERWISE you cannot guarantee your support.
"The details can be worked out later on, but without that you have no leverage."
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She goes on to raise worries about Mr Johnson's popularity with party members, and a "dislike" of him by Daily Mail editor Paul Dacre and publisher Rupert Murdoch.
Ms Vine wrote: "Crucially, the membership will not have the necessary reassurance to back Boris, neither will Dacre/Murdoch, who instinctively dislike Boris but trust your ability enough to support a Boris Gove ticket.
"Do not concede any ground. Be your stubborn best."
The email was sent yesterday morning and accidentally copied in to a member of the public, the channel reported.
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A spokesman for Mr Gove told Sky News: “We don’t comment on private email exchanges or conversations.”
Earlier today, Theresa May, seen as Mr Johnson's main rival in the race to succeed David Cameron, ruled out doing any deal with the ex-Mayor of London.