When Theresa May became Prime Minister in 2016 her job was to successfully negotiate a withdrawal from the European Union.
However with that task getting trickier by the day - if not the hour - she is now also facing a confidence vote from her Tory colleagues in Parliament.
Forty-eight letters from MPs have been written to the chairman of the backbench 1922 Committee, Sir Graham Brady, triggering the vote.
Boris Johnson, Dominic Raab and Esther McVey have already refused to rule themselves out of a potential leadership challenge - while a number of other candidates could put themselves forward should she lose the vote or surprise everyone by calling it a day.
Here are the bookies' leading candidates to succeed her as Tory leader and take over the mantle of delivering Brexit:
Dominic Raab - Voted leave
The second Brexit secretary appeared to strike a good relationship with the EU's chief negotiator Michel Barnier but his stint ended swiftly once the PM released her proposed Brexit deal.
The MP for Esher and Walton quit his role in protest in November and quickly joined the growing chorus against May's deal.
At 44 years old, Raab is the youngest potential Conservative leadership candidate on this list (younger MPs could stand) and bookies have him among the favourites.
Boris Johnson - Voted leave
The 54-year-old MP for Uxbridge and South Ruislip was touted as a potential Tory leader even before David Cameron won the job in 2005 - and has been waiting in the wings ever since.
The leading leave voter pulled out of the race to replace Cameron in the wake of the 2016 EU referendum result after Michael Gove withdrew his support - and instead took up the role as foreign secretary in Theresa May's administration.
Since quitting that post he's become a leading critic of her Brexit strategy and is a shoo-in to run in any race against her this time.
While Johnson is very ambitious and would be expected to run, he lacks support among fellow Tory MPs and might not make the final two of a leadership contest.
Sajid Javid - Voted remain
Despite voting remain in the referendum and publicly supporting the prime minister ever since, Sajid Javid is a serious contender to take the top job in Government.
His profile has risen since he replaced Amber Rudd at the Home Office and, unlike her, he's changed his original position on EU membership, saying he now backs Brexit.
This has made the ambitious 49-year-old MP for Bromsgrove in Worcestershire among the bookies' favourites.
Amber Rudd - Voted remain
Javid's predecessor Rudd gained sympathy from some after she was forced to resign as home secretary over the Windrush scandal, which centred on a hostile environment policy brought in by May in her time at the Home Office.
Despite this, her support for the PM has not wavered and she was recently rewarded with another Cabinet job as secretary of state for work and pensions.
The 55 year old voted remain in 2015 and confirmed she would do so again if another referendum was called, making it hard to see her unifying the party.
On top of this, she has a very slim constituency majority, something that could turn-off colleagues.
David Davis - Voted leave
The original Brexit secretary resigned from his role after the now-infamous Chequers Cabinet meeting in which the PM presented her plan for Brexit.
The long-term leave advocate has been publicly critical of May's withdrawal agreement and is touted by his supporters as a natural candidate to return to help reinvigorate negotiations as prime minister.
The 69-year-old MP for Haltemprice and Howden has repeatedly insisted he does not want to be prime minister but is rated as a contender rather than an outsider.
Michael Gove - Voted leave
Despite being a prominent Brexiter from the get-go, Environment Secretary Michael Gove chose not to resign from Government over Theresa May's withdrawal agreement - unlike Raab and McVey.
His support for the PM was apparently rewarded with an offer to take over from Raab, but the 51-year-old MP for Surrey Heath is believed to have turned down the opportunity.
His consistent prominent support for Brexit makes him one of the bookies' favourites to bid to lead the country through the end of the process.
Jacob Rees-Mogg - Voted leave
Before the 2016 referendum Jacob Rees-Mogg was the relatively unheard of backbench MP for North East Somerset - now he is a Brexit powerhouse with huge grassroots support.
Despite his consistent criticism of Brexit negotiations the 49 year old always claimed to support the Prime Minister - until he recently joined other MPs by pursuing a vote of no confidence.
If he succeeds in helping to force a leadership contest, he could soon throw his hat into the ring to oust May.
Jeremy Hunt - Voted remain
If a leadership contest was triggered the 51-year-old MP for South West Surrey could be one of the few remain-voting challengers.
Hunt's long tenure as health secretary ended this year - finally taking him out of the firing line as a stand-out target of many NHS protesters - and he has faced far less public criticism in his role as foreign secretary.
As the occupier of one of the high offices of state, he's a natural to be considered to stand and the bookies rate his chances.
Esther McVey - Voted leave
The former secretary of state for work and pensions has refused to rule out a leadership bid and resigned from Government, along with Dominic Raab, in protest over the prime minister's proposed Brexit deal.
When asked on The Andrew Marr Show if she would run in a potential leadership race, she said she would "if people asked".
Despite being one of the few prominent leave-voting female Tories, bookies view the 51-year-old MP for Tatton as an outsider.