Pro-Brexit Conservative MP Jacob Rees-Mogg delivers a speech at a Brexit rally during the Conservative Party Conference in Birmingham, Britain, September 30, 2018. REUTERS/Toby Melville

LONDON (Reuters) - Senior eurosceptic UK Conservative lawmaker Jacob Rees-Mogg will submit a letter of no confidence in Prime Minister Theresa May on Thursday, a spokesman for the influential eurosceptic group of Conservative lawmakers he leads said.

A challenge is triggered if 48 Conservatives write such letters. May could be toppled if 158 of her 315 lawmakers vote against her.