Conservative MP Jacob Rees-Mogg, leader of the hardline pro-Brexit European Research Group of lawmakers, on Thursday requested a vote of no-confidence in the British prime minister.
The ERG leader handed in a letter to the party's parliamentary group in the House of Commons after a raucous session of parliament in which Theresa May sought to defend her draft Brexit agreement despite trenchant opposition.
"Regrettably, the draft Withdrawal Agreement presented to parliament today has turned out ot be worse than anticipated and fails to meet the promises given to the nation," he wrote in the letter to 1922 Committee chairman Graham Brady.
"Therefore, in accordance with the relevant rules and procedures... this is a formal letter of No Confidence in the Leader of the party."
If 48 Conservative MPs write similar letters then a leadership contest is triggered.
Although other MPs have already sent letters, all eyes were on Rees-Mogg given his influence over Brexit supporting MPs.
Only 1922 Committee chairman Graham Brady knows how many letters have so far been submitted, with speculation mounting that the threshold may be close following the publication of May's Brexit divorce deal.
Rees-Mogg on Wednesday rejected the deal, saying it threatened the integrity of the United Kingdom, prevented Britain from striking free-trade deals with the rest of the world and made the country a "permanent rule taker" from Brussels.
If May wins a leadership contest, she will remain in office and cannot be challenged again for a year.
If she loses, she must step down and cannot stand in the leadership election.