Senior Conservatives have denounced Theresa May as too weak to unite her cabinet and run an effective government after she refused to sack her foreign secretary, Boris Johnson, for breaking ranks over Brexit.

Johnson infuriated Tory MPs on both sides of the European debate when he spelt out his own personal vision of a hard Brexit in a newspaper article, only days before May is expected to outline possible areas for compromise with Brussels in a speech in Florence.

Downing Street insisted that May still had full confidence in Johnson, although he had not informed her of the content of his article. Writing in the Daily Telegraph, Johnson insisted that the UK must not pay any money to the EU for access to its markets after Brexit and made no mention of a transition period after 2019 to avoid a cliff-edge for UK businesses.

May is now understood to back a transitional deal that could involve payments to the EU for access to its markets during a period of two to three years after Brexit, and to accept that the UK will need to follow the single market rulebook in that time.

Johnson’s article was seen across the Tory party as the start of a leadership campaign to replace May, in which he would position himself as the champion of a clean break with the European Union, in contrast to May and the chancellor Philip Hammond who are pushing for a gradual exit.

Several former Tory ministers said yesterday it was clear that Johnson was setting out his leadership stall and tempting May to sack him, in the knowledge that she had not got the authority to do so.

One said it was blindingly obvious that May should dismiss him, but it was doubtful that she would, because she was too weak: It is completely disgraceful. You do not write an article like that without consulting the prime minister and your cabinet colleagues. It is a complete abdication of cabinet responsibility. This is all about Mr Johnson, Mr Johnson, Mr Johnson, not about the interests of government or the country.

Another MP said Tory colleagues would be writing to the whips demanding that Johnson be fired because he was a law unto himself and a liability. He is deliberately tempting May to sack him but the awful thing is that she is too pathetically weak to do so. So we have a cabinet openly at war on the most important issue of the day and that is what we have to live with.

Johnson’s dramatic intervention is clearly intended to win support from Brexit hardliners in the party. The Tory MP Crispin Blunt, who was ready to back Johnson for the Tory leadership last year, came to his defence, stating that the article merely repeated government policy. But another MP said: If there was a chance of me supporting him one day, it’s gone now.

Ruth Davidson, the Scottish Tory leader and a long-standing critic of Johnson, made clear her disapproval of the timing of his article, the day after the Parsons Green bomb in London.

On the day of a terror attack where Britons were maimed, just hours after the threat level is raised, our only thoughts should be on service, she tweeted. In Brussels the response was immediate and furious. The Italian leader of the socialist bloc, Gianni Pittella, likened the tone of the intervention to that of Donald Trump.

Pittella said: Boris Johnson is embarrassing his country once again by repeating the lies of the Leave campaign. He is jeopardising the Brexit negotiations by threatening to turn the UK into a low-regulation economy. And he insults the intelligence of the British people with his tub-thumping jingoism. It is more in keeping with Trump Towers than Whitehall.

Many believe May will leave Johnson in place until after the Tory conference in Manchester, which begins in two weeks, and then dismiss him in a subsequent reshuffle. Some speculated Johnson could fuel the crisis at the top of the party by staging a dramatic resignation after May’s Florence speech, in the hope of using the conference to rally the hard-Brexit wing of the party behind him.

In an interview with the Observer, Sir Vince Cable, the Lib Dem leader, whose party conference opened in Bournemouth on Saturday said: He clearly thinks that Theresa May is on the verge of a U-turn which would lead to a transitional deal that would keep us in the single market.

Boris Johnson sees this as his chance to bag the top job, so is pushing for a far more extreme Brexit. This might play well with hard-right Conservative MPs, but would be a disaster for the UK economy.