SAM Galbraith, Scotland's environment minister, is expected to announce his resignation from the Scottish Executive today.
Mr Galbraith, one of the longest living survivors of pioneering lung transplant surgery, is retiring from front-line politics ''to protect his health''. He told friends he was ''sad to go and regretted it''.
He is likely to relinquish his ministerial post today, and his Scottish parliamentary constituency of Strathkelvin and Bearsden in a matter of weeks, whenever the date of the general election is confirmed.
Mr Galbraith, 55, has resigned in time for his constituency to hold the by-election for the Scottish parliament on May 3, to coincide with what is still the expected date for the general election, when he would automatically have left his role as a Westminster MP.
He intended to tell his constituency party last night. His successor as candidate for Strathkelvin and Bearsden MSP will be able to defend a majority of 12,121.
In an interview with The Herald, he said: ''I wanted to go to the back benches after two years, but that is not now possible. Scottish MPs and MSPs are under a great deal of pressure. It's very hard, you've got to be around and doing things.
''I have to get fit. I've not had time to take enough exercise. I need to keep swimming and have a lunch-time walk. If I had stayed at the parliament, that would not be possible.''
Mr Galbraith's resignation, which will cause little surprise among his closest colleagues, who have watched him battle bravely to undertake a daunting agenda, will spark the first significant reshuffle for Henry McLeish, the first minister.
Mr McLeish, never an ideological soulmate of Mr Galbraith, will nonetheless miss Mr Galbraith's experience of the political process, and the civil service. Mr Galbraith was one of only a handful of Holyrood politicians who had ever held parliamentary office.
Alistair Darling, social security minister and a close friend, said: ''In Scottish politics, Sam is an exemplar. He brought colour and in the Commons when he spoke on health, people always listened. He seems not to give a damn, but he does give a damn.
''Sam is a rare breed in politics. He had a distinguished career and a worldwide reputation. He gave it all up because he believed in social justice.''
The Strathkelvin and Bearsden MP, who snatched the constituency from the Tories in 1987, will become the first Labour minister to leave the Scottish Executive, and only the second MSP to trigger voluntarily a Scottish parliamentary by-election.
Ian Welsh, Labour member for Ayr, stood down in 1999, citing family pressures. The only other by-election for Holyrood came after the death of Donald Dewar, Scotland's late first minister.
Mr Galbraith's decision to go now marks a final break with the Dewar cabinet and will permit Mr McLeish to make further changes to his line-up. Alasdair Morrison, the Western Isles MSP and junior minister, is regarded as a possible ministerial replacement.
In 1998, Mr Dewar insisted that Mr Galbraith, a former Scottish health minister before health was devolved from Westminster to Holyrood, took on the education portfolio. Although the former neurosurgeon would have preferred to carry on with health, he knuckled down, loyally supporting Mr Dewar throughout.
Mr Galbraith's tenure at education was marred seriously last summer when he took responsibility for the shambles at the Scottish Qualifications Authority. Although he was not directly responsible for the SQA, he refused to pass the buck.
He said: ''When the music stopped, I was holding the parcel. It was not damaging for me personally but it was, quite wrongly, damaging for the executive.''
Given the environment portfolio when Mr McLeish took over the reins of power, he hugely enjoyed it. A lover of outdoor pursuits and an expert mountaineer, he was ''passionately keen'' on environmental issues.
Mr Galbraith stoutly defended the inclusive nature of Holyrood. ''It's been a success, and has involved more people in politics. It is beginning to settle down, and now has to learn to stand up to pressure groups and develop the ability to say no.''
He continues to argue with friends and former colleagues in the medical profession who criticised him for deserting his highly specialised surgery field to pursue a life in politics.
He said: ''I always missed medicine but I wanted to help get a Labour government elected. That's why I did it. I was not in politics for a career - I had one - but to make sure that everyone had the chances that I had.''