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Boris Johnson and his wife Marina Wheeler today announced they are to divorce after 25 years of marriage.

In a joint statement, the couple revealed they took the decision “several months ago” and that they were still friends.

The revelation did not appear to derail plotting among Mr Johnson’s allies to oust Theresa May from the Conservative leadership and try to install him as Prime Minister.

One supporter said the “toxicity” of Mrs May’s Chequers proposals for Brexit, which caused Mr Johnson to resign as foreign secretary in July, was a bigger factor in the minds of most Conservatives than the state of his marriage.

Mr Johnson and his wife, both 54, said in their statement: “Several months ago, after 25 years of marriage, we decided it was in our best interests to separate.

"We have subsequently agreed to divorce and that process is under way. As friends we will continue to support our four children in the years ahead. We will not be commenting further.”

The announcement came within hours of a report in The Sun that they were no longer living together. The newspaper said Ms Wheeler had accused Mr Johnson of another affair and that she was “finished with him”.

One Tory MP claimed it was a case of “shovelling the shit out of the way for the leadership bid”, according to one report. However, family friends disputed this, saying the couple had been preparing to make the announcement anyway ahead of court papers being made public shortly.

They were understood to have separated while he was Foreign Secretary and living in his grace-and-favour apartment at 1 Carlton Gardens, near Trafalgar Square. Upon his resignation, Ms Wheeler helped him move house.

Politically, the announcement appeared to be a bombshell that could potentially weaken Mrs May’s rival.

However, several MPs said they thought it would make little difference unless Ms Wheeler or another woman decided to attack his character in public. “This is hardly surprising to people,” said one. One supporter claimed: “You will start to see daily calls for Theresa May to stand down and an increase in the pressure for her to go.”

A minister who does not support Mr Johnson said opposition to Chequers among Conservative activists was so strong that Mr Johnson could achieve enough momentum to force a confidence vote in Mrs May. “I believe there is close to the number of letters needed to bring a vote,” the minister said.

Privately Cabinet ministers are deeply worried about the “toxicity” of the Brexit proposals. A straw poll of Tory activists by the ConservativeHome website yesterday gave Mr Johnson a huge lead over Cabinet rivals to be the next Tory leader.

Mr Johnson has had several well-publicised extramarital relationships in the past and reportedly fathered a child while Mayor of London, prompting a temporary separation with Ms Wheeler.

He married first wife Allegra Mostyn-Owen in 1987 and Ms Wheeler in 1993. He had a four-year affair with the writer Petronella Wyatt and had to resign as shadow arts minister when the Tory leader at the time, Michael Howard, felt he had not told the whole truth about the liaison. He was then said to have had affairs with journalist Anna Fazackerley in 2005 and art consultant Helen Macintyre in 2009.

Cambridge-educated Ms Wheeler is a senior human rights lawyer and QC who backs Mr Johnson’s views on Brexit.

A friend of Mr Johnson told The Sun: “I know they will always be incredibly close. He doesn’t do anything politically without consulting her.”