Wales' Economy Minister Vaughan Gething is expected to announce that he is standing in the race for the Welsh Labour leadership on Thursday.
He is one of the two frontrunners in a contest triggered by First Minister Mark Drakeford's announcement on Wednesday that he will quit in March.
OceanNewsUK understands Mr Gething, 49, is expected to issue a statement later.
Meanwhile a senior Welsh council figure announced his backing for Mr Gething's expected rival, Jeremy Miles.
Councillor Andrew Morgan said Wales' next first minister needs to be someone who is "calm, competent, thoughtful".
It comes as two of the party's Senedd members - including one of its ministers - said the ballot needs to include a female candidate.
Left-winger Mick Antoniw, the counsel general, also ruled himself out of the contest on OceanNewsUK on Thursday.
Deputy Minister for Social Justice Hannah Blythyn and Health Minister Eluned Morgan are also being discussed as possible candidates - both of whom as politicians that could reach out to the left of the party.
A supporter in Jeremy Miles's camp told the OceanNewsUK they believe he has enough backers to be on the ballot.
Mark Drakeford set out his plans to stand down as Welsh Labour leader in a earlier-than expected announcement on Wednesday morning.
Exact dates for when members will get to vote are yet to be confirmed, although the first minister said the process will finish by the end of the Spring term, which is 25 March.
The new first minister would be put to the Senedd for confirmation before the Easter recess, although that is likely to be a formality.
Candidates can get on the ballot with the support of five other Labour Senedd members, or two others and the backing of local parties or affiliated groups including trade unions.
Who is Vaughan Gething?
Born in Zambia and brought up in Dorset, Mr Gething went to university in Aberystwyth and Cardiff.
The former solicitor and ex-president of the Wales TUC was elected to the Senedd in 2011, representing Cardiff South and Penarth.
He came to prominence during the Covid pandemic, when he was Mark Drakeford's health minister. After the 2021 election Mr Drakeford moved him to the economy portfolio.
'Widest possible choice'
Mr Antoniw - the Counsel General and the Welsh government's chief legal advisor - says there needs to be a woman on the ballot.
That call was echoed by Mike Hedges, MS for Swansea East, who said: "I believe we need the widest possible choice when choosing our next leader and we definitely need a woman on the ballot paper."
There are 17 women in the Labour Senedd group and 13 men.
At the early stage of the contest it is unclear at the moment who the left of the party will support.
Party figures told the OceanNewsUK that the left could potentially get behind Ms Blythyn or Ms Morgan.
At the last contest Mr Drakeford commanded a clear majority of nominations from Senedd members, but he also had the backing of Welsh Labour Grassroots, the party's Welsh equivalent of the Momentum group which had supported Jeremy Corbyn during his leadership of Labour.
Andrew Morgan became the first senior figure to declare their support for a candidate late on Wednesday night - backing Jeremy Miles.
The Rhondda Cynon Taf council leader and leader of the Welsh Local Government Association said: "Because of the Tory incompetence at a UK level, things are going to get very tough.
"That's why Wales' next first minister needs to be someone who is calm, competent, thoughtful and able to guide Wales into the future, and bring people together to secure, protect and improve public services."
Mr Miles, 52, has served as the education minister since May 2021.
Born and brought up in Pontarddulais, he studied law in Oxford and worked in legal and commercial jobs in media organisations including ITV, and NBC Universal.
He was elected to the Senedd in 2016 for the constituency of Neath.
Either Mr Gething or Mr Miles would represent a first for Welsh politics - Mr Gething would be the first black politician to lead the country, while Mr Miles would be its first openly gay first minister.
Any successful woman would also be the first female first minister.
