Arlene Foster will quit the Democratic Unionist Party (DUP) when she stands down as Northern Ireland's first minister, BBC News NI understands.
Sources close to her said she thinks it is no longer the party she joined and it is moving in a different direction.
Mrs Foster is to resign as DUP leader on 28 May and end her tenure as first minister at the end of June.
On Thursday, Agriculture Minister Edwin Poots declared his intention to stand for the party leadership.
By Friday evening five of the party's Northern Ireland Assembly members and one of its MPs had publicly announced their support for his campaign.
No contact from colleagues
Mrs Foster announced her resignation after about 80% of the DUP's Stormont and Westminster ranks signed a letter of no confidence in her leadership.
Speaking on Friday, she said that none of her colleagues who signed the letter of no confidence had spoken to her since then.
She also said that some of her colleagues - including some "good friends" - had not signed the letter.
She said she would wait until she stepped down as first minister before making public her decision about whether she would remain in the DUP.
Mrs Foster has led the DUP since December 2015.
The following month she was appointed first minister, becoming the first woman and the youngest person to hold both jobs.
Sir Jeffrey Donaldson is expected to enter the DUP leadership race this weekend.
Edwin Poots has been quick out of the blocks with his bid and has been building support - all of his backers so far were always likely to take his side.
What will be key will be the floating voters among the party members entitled to have a say in this contest.
It is understood that Sir Jeffrey is confident he will get his fellow DUP MPs to support his campaign.
But he has lots of work to do to get the remainder of the Stormont ranks into his corner.
In her resignation statement on Wednesday, she said she was preparing to leave the political stage.
BBC News NI understands that means not only quitting as a Stormont assembly member but severing her ties with the party she has led since December 2015.
On Thursday she informed her constituency association in Fermanagh and South Tyrone of her decision.
'Engagement and debate'
Questioned on Friday about whether she intended to become a member of the House of Lords, she said that was a matter for other people to determine.
Asked about the future of the DUP, she wished her successor and the party well.
Mrs Foster has endured several major challenges during her time at the helm of the party, including division among members on social issues and significant recent pressure resulting from the fallout from Brexit, which the party supported.
The post-Brexit arrangements for Northern Ireland - which have included a trade border being imposed between Great Britain and Northern Ireland - have been deeply unpopular with the party's core voters.
BBC News NI understands that DUP officers will meet on Tuesday to discuss the timetable and the procedure for electing a party leader.
Announcing his desire to succeed Mrs Foster, Mr Poots said he was looking forward to the "engagement and the debate" in the leadership contest.
He has secured the backing of senior Stormont assembly members, the first of which was Christopher Stalford, who said his friend had the "experience and talents to take our country and our party forward".
Others DUP politicians who have declared their support for Mr Poots include assembly members Jonathan Buckley, Paul Frew, Paul Givan, Mervyn Storey and the MP Paul Girvan.
Winning the support of a majority of the party's 27 members of the legislative assembly (MLAs) will be crucial to Mr Poots' hopes of replacing Mrs Foster.
Only a small number of the DUP's membership - its MLAs and MPs - will get to vote in a leadership contest.