Theresa May would like the Tories to talk about anything but Brexit. She's about to be disappointed

The only policy the Prime Minister and her Brexiteers are agreed on is Boris Johnson's position on cake – having it, and eating it

Boris Johnson and other senior Brexiteers have recently been slapped down by the PM for going off-piste with remarks on the single market (PA)

When Theresa May and her closest advisers looked ahead to the Conservative Party conference after she became Prime Minister, her initial instinct was to largely avoid the biggest issue facing her government – Brexit. She would prefer to concentrate on her domestic agenda, such as her plans to help “just managing families” and a new industrial strategy.

On reflection, May realised that her “Brexit means Brexit” soundbite is already so past its sell-by date that it has turned mouldy. When the Birmingham conference debates “making a success of Brexit” on Sunday, I suspect she will be forced to tell us more about what it will mean. Her domestic policies will then dominate her keynote conference speech next Wednesday.

After 100 days of drift since the EU referendum, May’s refusal to give a “running commentary” on Brexit led her to slap down the Cabinet's “Three Brexiteers” – Boris Johnson, David Davis and Liam Fox – when their off-the-cuff remarks were taken by some to be Government policy.

Her approach has left a vacuum which hardline Tory Eurosceptics have used to push us towards a hard Brexit. They are never satisfied: they won the war on 23 June but still fear they will lose the peace – that May will opt for a soft Brexit with closer EU links than they can stomach. They are not yet convinced that she is committed to the “global free trade” espoused by Fox in his speech on Thursday. Four pressure groups have already been set up to hold May’s feet to the fire.

The soft-Brexit camp is outnumbered in Tory land, but Kenneth Clarke, George Osborne and Nicky Morgan have highlighted the dangers of sleepwalking to a clean break without thinking through the consequences for British companies, investment and jobs. Business people who raise such concerns with ministers backing a hard Brexit are told dismissively: “We’re going to do it.”

Philip Hammond, the Chancellor, who is batting for the City of London and other industries, has crossed swords with the Three Brexiteers, but it is May who chairs the crucial Cabinet committee and will call the shots. And even her ministers don’t yet know what they will be; a Cabinet “Brexit brainstorm” at Chequers was inconclusive.

May surely knows she needs to get a grip. If she does not, the danger is that the lively Tory conference fringe would be dominated by a divisive rerun of the referendum, with Leavers calling for a hard Brexit and Remainers a soft one. Which, of course, the media would lap up in the absence of a clear lead from the top.

Characteristically, May is immersing herself in a mountain of paperwork about the UK-EU relationship in deciding her negotiating goals. Some Tory MPs hope she will use the conference to announce when she will trigger Article 50 of the EU’s treaties, which will start a two-year formal negotiation.

It would hardly be a surprise if she said it will happen in January or February. The UK could then leave just before the next European Parliament elections in 2019, when a new European Commission will also be formed. Crucially, Britain would be out of the EU before the 2020 general election that May still has in mind – despite pressure from some Tories to capitalise on Labour’s weakness and go early, an idea that will be debated endlessly in the conference's margins.

A promise to invoke Article 50 early next year would give Whitehall a deadline, forcing it to end the skirmishing between departments and get its act together. The Three Brexiteers need to do the same. What some officials have dubbed “the three egos” need to become the “three amigos”.

May could tell the Tory conference more about her negotiating demands. Although aides say she does not want to declare her hand to the 27 EU countries, she can’t do that when she doesn’t have one. Whitehall insiders whisper that everything will stem from the PM’s bottom line on immigration.

Bizarrely, Johnson has dismissed as “complete baloney” an “automatic trade-off between what they call access to the single market and free movement”. But of course there will be; it’s in the EU’s treaties.

The Prime Minister could emphasise that her most important “red line” is be curbing EU migration, to deliver the people's instructions in the referendum That would play well with the Tory faithful; a survey of party members by the ConservativeHome website found that 78 per cent support a hard Brexit outside the European single market and with full control of migration.

The few clues so far suggest May agrees with Johnson on one thing: his policy on cake, which is “pro having it and pro eating it”. She will aim very high, for single market benefits plus controls on EU migration. It’s an opening bid that EU leaders will reject, despite the naive optimism of the Three Brexiteers. No wonder that some Whitehall officials fear a messy and disorderly exit. May needs to bring some order to prevent chaos.