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.
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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