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 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.”

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Reports suggest that around half a dozen of Mrs May’s senior ministers could be axed or moved, with Number 10 sources indicating that the more junior ministerial appointments would continue into Tuesday.

The most senior members of the Government – Chancellor Philip Hammond, Foreign Secretary Boris Johnson, Home Secretary Amber Rudd and Brexit Secretary David Davis – will all remain in their present posts.

Cabinet meetingTheresa May could choose to bring junior ministers into the Cabinet (Leon Neal/PA)

One of Mrs May’s first appointments is likely to be a replacement for Damian Green, who was forced to resign last month after admitting he lied over allegations that pornographic material was found on his Commons computer during a police raid in 2008.

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As first secretary of state, he was effectively deputy prime minister, standing in for Mrs May at PMQs and playing a key co-ordinating role behind the scenes chairing a series of cabinet committees.

Health Secretary Jeremy Hunt has been seen a favourite for the job but the Prime Minister may find it difficult to move him in the midst of an NHS winter crisis which has seen tens of thousands of operations cancelled.

Former first secretary of state Damian Green Former first secretary of state Damian Green (David Cheskin/PA)

All ministers being appointed to posts in the Cabinet or having their existing positions confirmed are due to go to Downing Street to speak with Mrs May over the course of the day.

The Conservatives deleted a tweet congratulating Chris Grayling on being appointed party chairman.

Tory MPs Rebecca Pow and Sheryll Murray also shared and then deleted the image initially posted by the party’s official account congratulating Mr Grayling.

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Neither Downing Street nor the Conservatives confirmed his appointment and a source close to the Transport Secretary said had had not been seen by the Prime Minister at the time the tweet went out.

Immigration Minister Brandon Lewis was seen entering 10 Downing Street through the famous black front door, fuelling speculation that the Great Yarmouth MP is being appointed to a Cabinet-level post after three-and-a-half years in lower-ranking ministerial jobs.

Immigration Minister Brandon LewisImmigration Minister Brandon Lewis arriving in Downing Street (Stefan Rousseau/PA)

James Cleverly was seen entering Number 10, fuelling speculation that he may land a senior role – which would be a rapid promotion from the backbenches after being elected MP for Braintree in 2015.

In his letter of resignation to Mrs May, Mr Brokenshire said he expected to be able to return to work “relatively quickly” after his operation.

But he said he recognised that he would not “be able to give the effort, energy and complete focus needed at this important time” and therefore believed that “the right thing at this time is for me to stand down from my current responsibilities”.

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In response, Mrs May said it was “absolutely right that you should put your health first, for your sake and that of your family”.

In an apparent signal that he may be in line for a return to Government once he has recovered after surgery, the PM said Mr Brokenshire would have continued to do “a good job” had it not been for his illness, adding: “I very much look forward to working alongside you again when you are back to full health.”

Sir Patrick McLoughlinSir Patrick McLoughlin confirmed he was leaving the Cabinet after eight years (Stefan Rousseau/PA)

CCHQ political director Iain Carter sent the image appearing to confirm Mr Grayling’s appointment as chairman to a majority of the party’s MPs in a WhatsApp message, before deleting it and saying it was sent in error, a Tory source said.

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Sir Patrick confirmed he was leaving Mrs May’s top team.

Asked if he was disappointed to be leaving the Cabinet, he told Sky News: “I’ve been in the Cabinet eight years. I have had a very good run and I enjoyed it immensely.”