Communities Minister Malcolm Chisholm has resigned after voting with the SNP over the Trident nuclear weapons system.
The BBC Scotland news website looks back over his political career.
Malcolm Chisholm has resigned twice
Trident is not the first issue on which Malcolm Chisholm has come into conflict with his party bosses.
The 57-year-old Labour MSP has also spoken out over Iraq in recent years, and back in 1997 he became the first minister to quit Tony Blair's new government at Westminster.
On that occasion it was his opposition to plans to maintain Conservative cuts to single-parent benefits which saw him stand down as a junior Scottish Office minister.
And in 2005, he described the controversial UK Government policy of dawn raids to remove failed asylum seekers as "totally unnecessary, heavy-handed and over the top."
The softly-spoken former English teacher had begun his parliamentary career in 1992 when he was elected as MP for North Edinburgh and Leith.
He was Labour's Scottish health spokesperson in opposition and was Scottish minister for local government, housing and transport before his 1997 resignation.
Two years later Mr Chisholm gave up his seat at Westminster to serve in the Scottish Parliament, when he was initially overlooked by then First Minister Donald Dewar.
However, Mr Dewar's successor, Henry McLeish, made Mr Chisholm deputy health minister in 2000.
He was promoted to the position of health minister by Jack McConnell when he took over from Mr McLeish in 2001.
The following year he won the Herald's Politician of the Year award.
In 2003 the first minister gave Mr Chisholm his backing after he spoke out over Iraq.
Jack McConnell accepted Mr Chisholm's resignation
The MSP for Edinburgh North and Leith backed the government's policy in a vote - but later said he regretted putting his loyalty to his party colleagues before his personal views.
Mr Chisholm said he had let himself down badly, adding: "I think ultimately that is an immoral thing to do."
At that stage Mr McConnell voiced his confidence in his health minister, who he said had a right to express his views on decisions made elsewhere.
Defence issues such as the war in Iraq and the replacement of Trident are reserved to Westminster.
"Malcolm Chisholm is an outstanding health minister who has done a fantastic job," said Mr McConnell.
However, little over 18 months later the first minister's opinion of Mr Chisholm seemed to have cooled somewhat.
He was switched to the communities brief in a reshuffle in October 2004, a move which was seen as a demotion.
The move came after criticism of his handling of the Scottish Executive's health cuts.
The chain of events which led to Mr Chisholm's latest ministerial departure was triggered by the publication of the UK Government's plans to replace the Trident nuclear weapons system.
Earlier this month, the prime minister outlined plans to spend up to £20bn on a new generation of submarines, although he suggested that the number may be reduced from four to three and the number of nuclear warheads cut by 20%.
A few days later Mr Chisholm spoke out on the subject, telling the BBC that he was against renewing Trident.
He said: "There may have been an argument for it five years ago, but I don't think it does apply in the modern world.
"I think we ought to try and get rid of the weapons we have through multi-lateral disarmament rather than encouraging proliferation through new investment in armaments."
Mr McConnell said then that he respected Mr Chisholm's opinion.
'Step too far'
The first minister had earlier told the Scottish Parliament that it was "legitimate" for MSPs to express their views.
He said he expected people inside his own party, and inside the executive, to "speak with their conscience and their mind".
But while speaking out against nuclear weapons was tolerated, it is thought that voting against the party line was a step too far.
Mr McConnell said he understood Mr Chisholm's position and had accepted his resignation.
He added that he was "very grateful" for Mr Chisholm's "substantial contribution to devolved government in Scotland".
Mr Chisholm is married with two sons and a daughter. His interests outside politics include reading, cinema and football.