Ms Alexander resigned on Saturday amid a long-running controversy over donations to her Labour leadership campaign.

The move came as a veteran Scots MP indicated he plans to step down from his seat for health reasons, raising the prospect of another by-election in the wake of Labour's defeats in Crewe and Nantwich and in Henley.

Ms Alexander's departure presents the party in Scotland with its second successive leadership campaign over the summer Parliamentary recess. During that period, Cathy Jamieson, deputy for the last eight years, will assume the role of acting leader.

Possible contenders in any battle to succeed Ms Alexander include the former health minister Andy Kerr, former communities minister Margaret Curran, Iain Gray, a minister in the first Scottish parliament who lost his seat in 2003 but got back to Parliament in 2007, and possibly Ms Jamieson herself.

Ms Alexander's resignation came three days after Holyrood's Standards Committee voted to recommend she be barred from Parliament for one day in the autumn for failing to promptly declare donations to her leadership campaign last summer.

Ms Alexander said the row had become a "distraction" from the real issues and that she has been the target of a political witchhunt.

Ms Alexander's resignation was confirmed shortly after the emergence of the likely resignation of Labour's Glasgow East MP, David Marshall, 67, for health reasons.

Labour said he had "indicated" to local activists that he was planning to stand down.

Prime Minister Gordon Brown heaped lavish praise on Ms Alexander, thanking her for her work in rebuilding the Scottish Labour Party since last year's elections.