Northern Ireland Secretary James Brokenshire is understood to have resigned from the UK Cabinet position due to ill-health.
Northern Ireland Secretary James Brokenshire has quit the Cabinet in the first of what are expected to be a number of changes as Theresa May reshuffles her top team.
The Northern Ireland Office confirmed his resignation, but gave no further details.
However, a source close to the 50-year-old minister said he had decided to stand down because he was facing major surgery within the next couple of weeks.
The Old Bexley and Sidcup MP is a close ally of Mrs May, having served under her for five years at the Home Office, and he was not among ministers who were predicted to go in the UK Prime Minister's first major reshuffle since she took office.
Explaining his decision, a source close to Mr Brokenshire said: "He has a small lesion on his right lung and is getting major surgery in the next couple of weeks."
Meanwhile, Theresa May has cleared the decks for a New Year revamp of her government.
The Prime Minister will seek to stamp her authority with a Cabinet reshuffle beginning today amid reports that up to six senior ministers could be axed or moved.
As MPs prepared to head back for Westminster following the Christmas break, she reaffirmed her intention to lead the Conservatives into the next general election.
Downing Street sources indicated the reshuffle was expected to be conducted over two days, with junior and middle-ranking ministerial appointments likely to continue into Tuesday.
It is likely to represent her biggest overhaul of her top team since she appointed her first Cabinet on entering No 10 in 2016.
She made only limited changes among her senior ministers following the election in June having seen her position badly weakened by the loss of her overall majority in the Commons.
It is thought her most senior ministers - including Chancellor Philip Hammond, Foreign Secretary Boris Johnson, Home Secretary Amber Rudd and Brexit Secretary David Davis - will remain in their current posts.
However, Education Secretary Justine Greening, Conservative Party chairman Sir Patrick McLoughlin, Business Secretary Greg Clark and the Leader of the Commons Andrea Leadsom are among those reported to be vulnerable.
Downing Street sources sought to play down the reports, describing them as "speculation" and "guesswork".
It is thought that Mrs May will take the opportunity to bring forward some more junior ministers, with Immigration Minister Brandon Lewis and Justice Minister Dominic Raab among those tipped for promotion.
It is unclear, however, whether she will announce a direct replacement for Damian Green who was forced to quit as her effective deputy after he admitted lying over the alleged discovery of pornographic material on his Commons computer during a police raid in 2008.
Health Secretary Jeremy Hunt had been widely seen as the favourite for the post, although reports have suggested that she may be reluctant to move him in the midst of an NHS winter crisis.
Shadow health secretary Jon Ashworth said yesterday: "If she promotes this Health Secretary tomorrow it's a betrayal of those 75,000 people in the back of ambulances."