Welsh Health Secretary Eluned Morgan will stand to replace Vaughan Gething as Welsh Labour leader and first minister.
Baroness Morgan has passed the threshold of six nominations to enter the contest after Mr Gething stepped down last Tuesday following the protest resignation of four members of his government.
She is the first person who has formally said they will run, with Labour politicians in the Welsh Parliament having until noon on Wednesday to decide who they will back.
Welsh Rural Affairs Secretary Huw Irranca-Davies is running as Baroness Morgan’s deputy.
After announcing her candidacy, the Welsh Conservatives called for the Senedd to be recalled from summer recess to appoint a new first minister.
The whole Senedd must vote for a new first minister and although the vote was set to take place on 18 September it may happen earlier if Baroness Morgan is the only nominee for Welsh Labour leader.
As Welsh Labour holds the highest number of seats in the Senedd, it is expected whoever wins the leadership race will become first minister.
If anybody else is nominated, then a contest will involve Welsh Labour members voting and a leader being announced on 14 September.
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First minister of Wales resigns
Baroness Morgan said: “I am proud to stand as a candidate to be the next leader of Welsh Labour, driven by a passion to serve the people and reset the relationship between the government and the people of Wales.
“I am delighted to be teaming up with my fantastic colleague, Huw Irranca-Davies, who shares my passion and experience in government and strong sense of public duty. Together, we are committed to putting Wales back on track.
“In the coming days, we will set out our priorities to improve public services, create better, greener jobs and empower our communities.
“Our focus will be meaningful change and building a brighter future for communities across Wales.”
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Eluned Morgan backed Vaughan Gething for the leadership in March. Pic: Senedd Cymru / Welsh parliament
Mr Gething has said he will stay on until his replacement is appointed.
He made history in March when he became the first black leader of any European country.
However, he continually faced questions over a £200,000 donation to his leadership campaign from a company owned by a man twice convicted of environmental offences.
Critics said Mr Gething’s leadership campaign should not have accepted the money, but the first minister said the donation was within the rules and was declared in line with party protocol.
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A row also ensued after he sacked minister Hannah Blythyn for allegedly leaking phone messages, which she denied and a newspaper later said she was not their source.
Plaid Cymru then pulled out of its co-operation agreement with Mr Gething’s government.
When he resigned, Mr Gething published evidence he said underpinned his decision to sack Ms Blythyn, including that a screengrab of the leak did not show her initials – suggesting it must have come from her phone.
But this came after the resignations of the ministers, which made his position untenable.
