LONDON (BLOOMBERG) - When Theresa May promoted a little-known, pro-Brexit lawyer to be attorney-general in July, she hoped he would be the right man to take on the legal complexities of the UK's divorce talks with the European Union.
Unfortunately for the prime minister - and her chances of striking a Brexit deal - Geoffrey Cox is proving to be too good at his job.
The government's top law officer is now leading the pro-Brexit revolt inside May's Cabinet, according to people familiar with the matter, who spoke on condition of anonymity.
In precise legal detail, Cox, 58, has warned his colleagues against giving in to the EU's demands over the future of the Northern Irish border - the key sticking point in negotiations.
His intervention was so decisive in Tuesday's Cabinet meeting that it led to a furious row and has slammed the brakes on Brexit talks, with the UK side divided over what to do next. Negotiators are running out of time to secure a deal and get it ratified in the European and British Parliaments.
The UK automatically leaves the bloc - with or without a deal - on March 29, 2019.
If no divorce is agreed, businesses will face widespread disruption from a hard and chaotic split, as the 21-month transition period to cushion the impact of Brexit will not come into force.
Progress now hinges on whether an acceptable compromise can be reached for the Irish border. Both sides say they want a key legal text - known as the backstop - to guarantee that there will be no need for customs checks on goods crossing the Irish border from Northern Ireland.
One person familiar with the Cabinet's deliberations described May's most senior ministers as divided into "the Geoffrey Cox Gang" and the pro-EU ministers, including her deputy David Lidington, who are rallying behind the premier and trying to give her the space to compromise on the issue.
One person said Cox is "confident, a good public speaker and debater", while another argued that his "forceful" interventions had changed the Cabinet debate on Brexit.
In the past two weeks, Cox has made critical arguments in two Cabinet meetings, warning that the UK risks being tied into the EU's so-called Irish backstop forever because of the nature of the treaty being proposed.
He's argued that while normal treaties can be altered over time, the kind of powerful guarantee envisaged for the backstop would leave the UK practically unable to exit the arrangements, according to three people familiar with the debate.
May was said to be on the point of signing up to a plan that would allow the UK to stay in the EU customs regime to avoid the need for checks at the Irish border.
Crucially, this arrangement would not have an end date, and there are now discussions under way on whether any other proposed mechanism to enable the UK to leave the backstop could ever be viable.
For pro-Brexit Conservatives - including Cox, Trade Secretary Liam Fox and Environment Secretary Michael Gove - breaking free from EU customs rules in the long term is a key prize of leaving the bloc.
Only if the UK is fully outside the EU customs union will it be able to strike free trade deals with countries elsewhere in the world, they say.
Cox brings 30 years' experience of courtroom legal arguments to his interventions in the debate. He asks May's officials the kind of precise and probing legal questions nobody else in the room can match, one of the people said.
On the Record
This leaves May in a bind. She needs to be able to make compromises to secure a deal with the EU, but it is politically and constitutionally fraught for her to ignore the legal advice of the attorney-general.
After the last two Cabinet meetings, Cox's words of warning are now written down in the official minutes and as such are a matter of government record.
Cox shot to prominence this month with a storming performance as the warm-up act to introduce May's keynote speech at the Conservative Party's annual conference. His rich voice and florid style - quoting Milton - won him fans on social media and even sparked speculation that he could harbour his own leadership ambitions.
Cox honed his oratory in the courtroom.
He has been a barrister since 1982 and was appointed Queen's Counsel - a high honour for top performers - in 2003. He lives in Devon, a rural district in southwest England, with his wife, Jeanie, their daughter and two sons.
A local man, his political passions before entering government included protecting cattle farming, bolstering rural communities and safeguarding the environment.
In February 2016, Cox announced he would be campaigning to leave the EU "after years and months of wrestling with my conscience" and the competing arguments for and against Britain's membership of the bloc.
"We do not need to be afraid of resuming full control over the government of our country," he said at the time.
"It is clear that our partners are unwilling to make the necessary adjustments to tackle the fundamental and deeply entrenched problems of the European Union."