Nicola Sturgeon will step down as Scotland's first minister after more than eight years at the helm.
The leader of the Scottish National Party is expected to make the announcement at a press conference organized in Edinburgh. Her departure is not expected to be immediate, allowing time for a successor to be chosen.
Sturgeon has been a member of the Scottish Parliament since 1999 and became deputy leader of the SNP in 2004.
She has been prime minister since November 2014, when she took over from Alex Salmond after losing the independence referendum. Sturgeon has led the SNP to a series of election victories at UK, Scottish and local level.
Last year the UK High Court ruled that Holyrood did not have the power to hold another independence referendum, a move which has been blocked by the UK government.
Sturgeon wants the SNP to fight the next general election as a de facto referendum, but there has been some opposition to the plan within the SNP. In addition, recent months have seen controversy over gender reforms, which have been blocked by the UK government.