As Theresa May prepares to take her flagship piece of Brexit legislation back to Parliament, one man more than any other has the power to frustrate the prime minister: her former cabinet colleague, Dominic Grieve. The former attorney general and ex-army reservist who chairs Parliament’s Intelligence and Security Committee is marshaling moderates with the aim of foiling Brexit hardliners. Tory insiders say he can do it. “It’s very hard to paint Grieve as an anarchist who’s trying to bring down the government, so Tories can support him,” said Philip Cowley, professor of politics at Queen Mary University of London. “And his legal expertise means his suggestions are taken seriously. He’s the ideal leader for a rebellion on this.” Their arena is the debate over the 66-page repeal bill. Lawmakers have so far tabled 142 pages of amendments, but many of those will fall by the wayside for lack of support. Conservatives are reluctant to back changes they feel could be used to stop Brexit altogether or amendments proposed by the opposition Labour Party that could destabilize the government. The 19 amendments proposed by Grieve, 61, are a different matter. The Oxford-educated son of a former Tory lawmaker has impeccable establishment credentials. One Conservative member of Parliament, speaking anonymously, described him as the most important man in Britain on Brexit, except no one has noticed. Another lawmaker agreed.  The point of the bill is to provide certainty for businesses that existing British laws derived from the EU will continue to apply after the U.K. has left the bloc in 2019. Alarm for May The danger for May is less that the legislation doesn’t make it through Parliament, as that its passage takes so long that business decides she’s unable to deliver certainty on the Brexit outcome and makes alternative plans. Very few Conservatives could support any amendment put forward by the Labour leadership, and it’s also unlikely they’d get behind changes proposed by rank-and-file opposition members of Parliament, a third Tory lawmaker said, adding that the aim is to get Labour to support changes proposed by Grieve and other Tories who aren’t part of the government. The MPs asked not to be named because of the sensitivity of the issue. Grieve is a passionate opponent of those Tories who want maximum distance from the EU. The former attorney general is also determined to protect the country’s constitution, describing the bill as an “astonishing monstrosity” for the power it gives ministers to change laws without scrutiny when EU rules are transferred into British statutes.  “He is knowledgeable about the law and the constitution and the point that he is making, I share,” said Steve McCabe, a Labour MP backing Grieve. “The government has taken this opportunity to try to change the way we make laws, not to simply transpose them.” Get it Right Many of Grieve’s amendments have already gathered significant cross-party support. They cover several areas: limiting those ministerial powers; keeping European human-rights law in British law; and forcing the government to put before Parliament another bill to approve its final Brexit deal. “The Brexit bill is a very important part of the process of leaving the EU, and we have to get it right,” Grieve said in a telephone interview. “The amendments I’ve tabled are designed to improve the legislation and make sure Parliament keeps control of it.” May has a majority in Parliament only with the support of the 10 MPs from Northern Ireland’s Democratic Unionist Party, making her vulnerable to rebellions on her own side. Several of Grieve’s amendments, including the one requiring a new bill before Brexit, already have the support of 10 Tories. Privately, one member of the government said they expected ministers would give way and accept most of Grieve’s amendments, a reflection of the fact that he was likely to win anyway if they went to a vote. But there was more reluctance around anything that could be used to delay Brexit. The government was supposed to start debating the next stage of the bill this week, but the schedule was pushed back, with the leader of the House of Commons, Andrea Leadsom, saying ministers need more time to consider their response to the flood of amendments.  No date has yet been given for the bill to reappear. A Labour lawmaker, speaking on condition he not be named, said his party’s expectation is that the bill will return to the Commons on Nov. 14. On Thursday in London, lawmakers jeered at Leadsom for failing to give a date on when the Brexit bill would next be debated. In charge of timetabling legislation, she instead gave them a history lesson on past legislative pauses. Her Labour counterpart, Valerie Vaz, had this to say in response: “No wonder the EU negotiating team think they are amateurs and want to talk to us.”