Finance and Local Government minister Sue Essex has announced she is to stand down at the next assembly elections.
Sue Essex has had a 25-year career in local and Welsh politics
Ms Essex, 59, has been the Assembly Member for Cardiff North since the first assembly elections in 1999.
She began a 25-year political career as a Cardiff councillor, later becoming the first woman to lead the council.
Her decision is likely to put further pressure on the Labour majority at the next assembly election with the Tories targeting her marginal constituency.
The next assembly elections are due to be held in 2007.
First Minister Rhodri Morgan said Ms Essex had made a "massive contribution" to assembly politics.
Ms Essex said on Friday: "Dedicating almost 25 years of my life to serving the people of Cardiff and then Wales has been an enormous privilege, with every day offering a new challenge and opportunity.
"The last eight years have been exciting times for Wales, with the establishment of our National Assembly, and I think people will see this as a period of success and delivery.
"In terms of achievements I was very proud as transport minister to deliver the free bus pass scheme for older and disabled people in Wales.
"During my time as both a councillor and AM I have always hugely appreciated the support I have received from my constituents in Cardiff North."
Ms Essex has served as minister for the environment, planning and transport as well as in her current role as finance minister.
First Minister Rhodri Morgan said he had tried to dissuade her from standing down "because she is such an excellent local and national politician, but I respect her decision. It is a matter for her and her family.
"She has already made a massive contribution at the assembly, particularly within the fields of planning and finance, both as a backbencher and a frontbencher, and I am sure she will continue to do so."