Karen Sinclair has been an AM since the assembly was founded in 1999
A former cabinet minister in the Welsh Assembly Government is to stand down at the next elections in 2011.
Karen Sinclair, who has served as Labour AM for Clwyd South for 10 years, told her constituency party on Tuesday she would not be seeking re-election.
Last year, Ms Sinclair was diagnosed with the bone marrow cancer myeloma and has been undergoing treatment.
First Minister Rhodri Morgan said Ms Sinclair had always "put others first and not herself".
Ms Sinclair served as business manager in the cabinet between 2003 and 2005, and was chief whip.
She said it had been a "great honour" to represent the constituency, adding: "The recovery from my recent illness is going very well, but I felt the time was now right to announce I will step down and tell my local party I will not seek re-election in 2011.
"I've been extremely proud to work with the Labour assembly government over the last decade, one that has been committed to social justice and to making a real difference to our local communities.
"We have done so much to help hard-working people and families since 1999 and I look forward to seeing this work being taken on by the next Labour leader."
She continued: "Having been an AM for a decade I am immensely proud of what I have achieved with my colleagues, not least having helped to take forward the regeneration of west Wrexham by chairing the Urban 2 programme in the area.
'Forthright and friendly'
"I am also very proud of the work I did in helping to clear the old Brymbo steelworks site and campaign to improve the lives of the deaf community across Wales."
Rhodri Morgan was "sorry" to hear Ms Sinclair would stand down, but said: "It is perhaps inevitable that the severity of her illness has taken its toll, and I am sure that she has made the right decision for both her family and her constituents in Clwyd South.
"Karen has always put others first and not herself, and I am sure she will continue to serve her constituents, as she has done since 1999, in the same forthright and friendly manner for the next 18 months."