Michael Gove has leapt to the defence of Boris Johnson after the emergence of a previously unseen newspaper column in which the Foreign Secretary backed staying in the European Union.
Mr Gove, who Mr Johnson apparently "wanted to punch" for effectively torpedoing his chances of becoming prime minister, insisted his Leave campaign ally acted on his "deep beliefs" when he decided to support Brexit.
The former justice secretary's comments take on added significance given the spectacular way he abandoned Mr Johnson's campaign to become Tory leader to have his own run at the top job.
It came as anxiety over the Government's apparent support of a "hard Brexit" outside the European single market reached new heights, with devolved leaders calling for their own bespoke deals with the EU, renewed cross-party calls for a vote on the negotiating strategy, and reports of splits within the Cabinet.
Observers say Mr Gove is widely regarded as persona non grata among Tories after his supposed "betrayal" of Mr Johnson after the pair campaigned closely together for Brexit.
But Mr Gove has now defended the Foreign Secretary, whose pro-EU article and desire to punch his former ally was revealed in a new book and published in the Sunday Times.
Mr Gove said the column has done "nothing to undermine the sincerity" of Mr Johnson's position, adding "quite the opposite".
He tweeted: "I am in no doubt Boris Johnson is and was 100% sincere and passionate about Britain leaving the EU - his conviction was clear.
"It is never easy to take a different line from your party leader - especially on such a big issue - but (Mr Johnson) followed his heart.
"(Mr Johnson) acted in accord with his deep beliefs - his motivation was to do what he thought was right for the UK long-term."
Mr Johnson has claimed the column was "semi-parodic".
He admitted he was "wrestling" with whether to back Brexit, but claimed it became "blindingly obvious" after he set out the arguments in two separate articles.
He is now seen as a backer of a hard Brexit, last week insisting the UK can get a trade deal that is "of greater value" to the economy than access to the single market, which he described as an "increasingly useless" concept.
But in the pro-Remain article, published in All Out War by Sunday Times political editor Tim Shipman, he suggested contributions to the EU budget may be a price worth paying for membership of the trading bloc.
Meanwhile, in opposition to a hard Brexit, Scottish First Minister Nicola Sturgeon said she believes Scotland could stay in the single market if other parts of the UK leave, promising detailed proposals within weeks.
And Northern Ireland's Deputy First Minister Martin McGuinness warned of economic "disaster" for the country outside the single market, with a knock-on effect on its neighbour Ireland.
He renewed calls for Northern Ireland to have special EU status.
Mr McGuinness told the Guardian: "Fifty-six per cent of the population voted to Remain and we could not have achieved that result without a substantial section of the unionist community voting for a future in Europe.
"The vote was not on sectarian lines, but on how destructive it would be politically, socially and economically.
"May says she is negotiating on behalf of the United Kingdom, but there is absolutely nothing united about a so-called United Kingdom. We don't agree. We see our future in Europe. Scotland sees its future in Europe."
Theresa May is seen as increasingly likely to pull the UK out of the single market due to her desire to regain full control over immigration, seen as incompatible with membership of the free trade zone by European leaders.
According to the Telegraph and the Times it has led to a rift in the Cabinet as Chancellor Philip Hammond argues against colleagues' policy ideas that would be incompatible with membership.
It is reported that Mr Hammond urged caution after Home Secretary Amber Rudd raised plans for a work permit scheme for skilled migrants last week.
Proponents of a "softer" Brexit may support plans the Financial Times claims are being discussed by the Government to continue paying some contributions to the EU in order to maintain "passporting rights" to allow City firms to continue trading across the continent.
On Sunday, pro-Brexit International Development Secretary Priti Patel rejected cross-party calls for the Government to spell out in a White Paper-style document whether it sees the UK in or out of the single market before beginning negotiations.
"If I were to sit down and play poker with you this morning, I'm not going to show you my cards before we even start playing the game," she told BBC One's Andrew Marr Show.