TO HIS colleagues, he was an unlikely assassin. A former Catholic priest who quit his vocation to pursue his political ambitions, David Cairns was a loyal Labour MP who had never voted against the government and had been rewarded with a ministerial post in the Scotland Office.
But last night, he became the most senior member of the party to call for Gordon Brown to go, after he was outed as the minister who, 24 hours before, had threatened to resign if Labour lost the Glenrothes by-election. That forced him to step down.
His resignation from the Scotland Office came as a surprise to those close to him: only 12 hours earlier, he had not been planning to quit.
However, The Scotsman understands that, after the Inverclyde MP's name was linked on newspaper websites and by the BBC to reports that one minister was on the brink of quitting, he felt he had no choice but to leave his post.
In doing so, Mr Cairns has dealt a severe blow to Mr Brown, and his resignation could become the trigger for a chain of events that may lead to the Prime Minister's downfall.
Yesterday morning, Labour's ruling National Executive Committee vetoed a demand by rebels that leadership nomination forms be sent to all MPs before next week's party conference.
However, speculation was rife at Westminster that more resignations could come at ministerial level, but not until after the Glenrothes by-election, expected to be in early November.
Mr Cairns was swiftly replaced at the Scotland Office by Ann McKechin, the Glasgow North MP who branded 12 of her colleagues "stupid" at the weekend for calling for a leadership contest.
Her predecessor said he had left the government with a "heavy heart". He insisted he was not part of a wider plot, nor was he part of some orchestrated campaign to depose Mr Brown.
He admitted he would prefer someone else to take over as Labour leader – believed to be David Miliband, the Foreign Secretary – but refused to name that person.
"I am not some sectarian figure who is weaving around, dripping poison," he said.
Mr Cairns had told the 12 rebels who called for leadership contest nomination forms to be sent out that it was a "mistake" to do so now.
He said he was disappointed the names of the dozen – who include Siobhain McDonagh, the former junior whip for whom he once worked and a former ministerial aide to John Reid – had somehow been leaked.
Mr Cairns said he had become disillusioned after colleagues were labelled "stupid" for calling for a leadership contest – a riposte aimed at Ms McKechin.
He said that his resignation had been "a matter of conscience" and insisted he was not acting on behalf of a more senior minister.
"I have got my own views as to who I think would be a preferable leader of the Labour Party, but I am not going to say who that is, because people would think there is some big plot and they have put me up to it," he said.
"Nobody put me up to this. I took this decision with tremendous reluctance and a degree of wretchedness that doesn't sit easily with somebody for whom loyalty is part of my DNA."
Mr Cairns said Mr Brown had been an "outstanding" Chancellor, and he nominated him as Labour leader last year in the hope that he would be equally successful as Prime Minister.
But he went on: "We are 20-odd points behind in the opinion polls and we are losing by-elections.
"To be so far behind a vacuous Tory Party, which has nothing to offer the people, and so far behind the SNP, which has betrayed the people of Scotland… means we have at least got to be prepared to ask ourselves the question."
He said it had become obvious that the National Executive Committee did not want even to face the "question".
"If people aren't prepared to reflect on the question, then the chances of finding the solution are remote," he said.
Mr Brown wrote to Mr Cairns accepting his resignation and said he was "disappointed" at his decision to quit the government. However, the Prime Minister warned that now was not the time for Labour to indulge in internal wrangling.
"As you know, the world is facing a time of economic upheaval. I believe it is vital that we, as a government and as a country, stand together in the face of these difficult times and concentrate all our efforts on helping the British people to come through them," he wrote.
"I am therefore disappointed by your decision to leave the government, and I do not agree with you that this is the time at which the Labour Party should be focused on internal debates."
The Prime Minister had lectured the Cabinet earlier yesterday on the need for unity and loyalty.
Two MPs, Ms McDonagh and Joan Ryan, have already lost government jobs and a third, Barry Gardiner, resigned after calling for a leadership contest.
During phone calls late on Monday and yesterday, Des Browne, the Scottish Secretary, attempted to deter Mr Cairns from becoming the most high-level casualty. But he failed.
The Scotsman understands that Mr Browne, who has the pressure of being Defence Secretary as well as running the Scotland Office, was "very upset" about Mr Cairns's departure.
A source close to Mr Browne said: "David is a very talented minister – it is a real loss for us. He (Mr Browne] was able to completely trust the political and administrative management of the Scotland Office to David. That's why he was promoted."
Tom Harris, the Glasgow South MP and a transport minister, said: "I'm really sad that he has decided to go. I think he was an absolute first-class minister. I hope he will be back in government at some point."
However, one of the MPs calling for a leadership contest, former minister George Howarth, last night said Mr Brown was now the most unpopular Prime Minister since Chamberlain was deposed as war leader in 1940.
And another senior MP and critic of Mr Brown, who did not want to be named, said Mr Cairns had been speaking for the majority of Labour MPs when he called for a leadership election.
"There are Cabinet ministers who agree with his sentiments, but the timing may not be quite right. The party machinery may be saying that there is nothing wrong, but they are either lying or in complete denial," the MP said.
Meanwhile, opposition parties seized on what they said was a sign of turmoil at the heart of the government. David Mundell, the shadow Scottish secretary, called for a general election, saying: "Scotland cannot afford another 18 months of a divided Labour Party, a weak Prime Minister and a UK government driven by self-interest rather than the national interest."
Angus Robertson, the SNP's Westminster leader, described Mr Cairns's decision as "a body-blow to Gordon Brown".
DAVID Cairns was born in Greenock and grew up to become a Roman Catholic priest. From 1991-94 he served in Scotland and south London.
From 1994-97 he worked as a director of the Christian Socialist Movement and secretary for Labour's socialist societies.
He was a researcher for Siobhain McDonagh, who was sacked as an assistant whip last week after becoming the first Labour MP to publicly demand nomination papers for a leadership election, and served as a councillor in the London Borough of Merton.
Legislation in 2001 enabled the former priest to stand for parliament and in 2005 he was returned as MP for the enlarged constituency of Inverclyde.
Political hurdles need some leaps of faith
IF GORDON Brown wants the rest of his time in office to remain peaceful, he has a series of obstacles to surmount.
He may have fended off a call for leadership contest papers to be issued, but the reprieve could be temporary.
• LABOUR CONFERENCE: From Saturday to Tuesday, the party will be in Manchester, where the Prime Minister will have to placate rebels. With Labour MPs meeting in person in large numbers for the first time since their lengthy summer break from Westminster, the potential for collusion and trouble is high.
• GLENROTHES: The by-election in Mr Brown's backyard is now crucial for his survival. If Labour loses he will be blamed, regardless of whether or not he plays a prominent role. Several ministers are believed to be ready to quit if the SNP wins.
• PRE-BUDGET REPORT AND QUEEN'S SPEECH: Mr Brown will have to come up with powerful antidotes to the economic woes facing the country, as well as unveiling an inspired legislative programme.
• RESHUFFLE: A bigger shake-up than originally planned could be in order if senior rebels need to be punished, while rivals may have to be placated. Some of his most senior Cabinet ministers are now virtually unsackable, however, as Mr Brown will not want his biggest threats to be ungagged and outside the government.
• MOTHERWELL AND WISHAW BY-ELECTION:
Jack McConnell, the former first minister, is due to take up his post as High Commissioner to Malawi next year. There will be little reason for Mr McConnell to stay in Scotland if Labour's leader cannot offer him a better long-term alternative. His departure will trigger a by-election, this time at Holyrood but nevertheless potentially damaging for the PM.
• EUROPEAN AND LOCAL ELECTIONS: If Mr Brown is still in place next spring, this will be his final major electoral test ahead of a general election.