First it ramped up the pressure on May to make a clean break from the EU, including fully withdrawing from the customs union. Then it led the pushback against the secret government analysis leaked to BuzzFeed News, which suggested Brexit will be bad for the UK economy under every scenario examined. Anna Soubry, the most outspoken of the Tory Remainers, claimed a clique of “35 hard ideological Brexiteers” are holding May to ransom and urged her to throw them out of the party. Newspaper reports claimed the ERG is ready to overthrow the prime minister if she crosses its Brexit red lines.

The Brexiteer caucus has done more to shape the UK’s Brexit policy than the official Labour opposition, the army of lobbyists employed by the City, big business, and trades unions, and the smaller group of committed Remainers on the Conservatives’ opposite flank. “They are the people who hold the whip hand in the party,” said a former senior 10 Downing Street official with close knowledge of the Brexit process.

But as the group’s visibility has dramatically increased, so have the questions about the extent of its support, its funding, and its contacts with government. Even in Westminster, it remains a mostly spectral organisation. A tight-knit inner circle, fronted since January by Rees-Mogg, coordinate discreetly in their offices in the House of Commons, working back channels to 10 Downing Street and the Brexit department. A wider network of supporters liaise through WhatsApp. The ERG has rarely been penetrated by outsiders, suffering only a handful of leaks. It has barely any presence online. There’s no register of its contacts with ministers and government officials. Even its membership is a mystery, with estimates varying wildly.

But now a new analysis by BuzzFeed News, based partly on unprecedented access to ERG sources, has identified about 70 MPs who are part of the informal network of committed Brexiteers — twice as many as Soubry claims, and at the upper end of newspaper estimates. We believe this figure to be conservative.

The analysis illustrates why the ERG has exerted such influence over the Brexit process — and why it is so feared by 10 Downing Street. The size of its parliamentary base, as calculated here, is more than five times May’s working majority in the House of Commons. The group has easily enough members to trigger a leadership contest if it turns against the prime minister, and enough to have a major influence on the outcome of that contest when it happens.

