Michael Gove's wife has urged her husband to play hardball with Boris Johnson before making any promises to support him in the Tory leadership contest, an email she accidentally sent to a member of the public revealed.
Sarah Vine said the Justice Secretary must secure a specific guarantee about his future before making any deal.
She also wrote that Paul Dacre - editor of the Daily Mail, which Vine writes a column for - and media mogul Rupert Murdoch "instinctively dislike" Johnson.
In an email to Gove and his team, which was passed to Sky News, she wrote: "Very important that we focus on the individual obstacles and thoroughly overcome them before moving to the next. I really think Michael needs to have a Henry or a Beth with him for this morning's crucial meetings.
"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.
"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. GOOD LUCK."
Sources close to Gove said the contents of the email were Vine's personal opinion.
It comes after Vine revealed that it had been "hard" to help her husband "make the right decisions in such a short space of time, on very little sleep and under such stressful conditions" following the Brexit vote.
In her Daily Mail column, she said voters had shown "incredible bravery" and she was now involved with implementing the will of the public.
"They ignored all the threats and lies, and voted according to their principles," she said. "Which, from where I'm standing, makes the result even more terrifying. Because, given Michael's high-profile role in the Leave campaign, that means he - we - are now charged with implementing the instructions of 17 million people. And that is an awesome responsibility."
Vine told how the Cabinet minister had gone to bed shortly after polls closed on referendum night but was woken by a mobile phone call in the early hours telling him the Leave campaign had won. He responded to the shock news by saying: "Gosh, I suppose I had better get up."
She revealed that "at the height of the hysteria" her son "led us all in a surprisingly therapeutic game of Monopoly".
The couple had been close friends of the Prime Minister and his wife Samantha but the campaign has strained the relationship.
In the column, however, Vine insisted: "David Cameron was not supposed to go. This was not what this referendum was about; that was not why Michael backed Leave."
A spokesman for Gove said: "We don't comment on private email exchanges or conversations."