A Labour MP is in secret talks with the SNP about leaving the party and defecting to the Nationalists, the Sunday Herald has learned.
The SNP hierarchy has had several high-level discussions with the politician, who is said to be unhappy with Tony Blair's foreign policy.
Nationalist leader Alex Salmond hopes to unveil his new recruit ahead of May's crucial Holyrood election.
The revelation is the latest blow for Labour as they attempt to hold on to power in the Scottish parliament.
Various opinion polls have found Labour lagging behind the SNP, whose leadership are treating the Holyrood poll as their best ever chance of winning an election.
The continuing crisis in Iraq, the UK government's support for renewing Trident and the cash-for-peerages controversy are thought to be harming Jack McConnell's chances of retaining power.
But Labour's election hopes are likely to deteriorate further with the revelation that one of their Scottish MPs has been having clandestine talks with the SNP about defecting.
The meetings, which took place in London, focused on the possibility of joining a party always opposed by the MP.
The MP is understood to be opposed to the government's decision to back the renewal of the Trident missile system. He or she is also believed to be unhappy about the current situation on Iraq.
A source close to the talks said: "Discussions have taken place between the SNP and a Scottish Labour MP. They have been on a what if' basis but the person is serious about defecting."
A defection would be the first time a Labour MP had switched to the SNP since Dick Douglas joined the Nationalists in 1990.
The SNP has also had talks with several Labour councillors about jumping ship. The Sunday Herald understands that Salmond and his colleagues have turned away six Labour councillors because the party doubted their motivation as potential defectors.
While it is good news for Salmond and the SNP, a high-profile defection would be a hammer blow for McConnell's re-election hopes.
The Scottish Labour leader is said to be frustrated that his party's chances are being harmed by events outwith his control in London. A walkout by a Labour MP would be seen as another distraction emanating from Westminster.
His supporters are also thought to be irked that his largely upbeat campaign message - making Scotland's education system the best in the world - is being drowned out by UK Ministers' attacks on independence.
McConnell yesterday admitted his party faced a struggle in its chase for votes with the SNP. Addressing Labour candidates in Edinburgh, he said: "We have a real fight on our hands. But the prize will be worth it. On May 4th, to have bucked every international trend for a mid-term election, to have rewritten the political textbooks, to have made history once again, and above all to have the opportunity to serve for four more years."
In addition, he admitted to feelings of "fallibility" as a politician: "I'm not going to apologise for being a human being with cares and concerns, passions and doubts."
He also used the speech to attack Salmond's credibility as a candidate for First Minister: "If you want snide comments, sarcastic laughter and playground mockery you know which leader to go to."
Salmond declined to talk about the defection talks last night.
An SNP spokesman said: "Clearly there are tensions within the Labour party, not least over the leadership's determination to foist a new generation of nuclear weapons on Scotland, however we are neither anticipating or claiming any immediate defections.
"The SNP is a welcoming and inclusive party and we would welcome anybody with Scotland's best interests at heart."Labour MP. They have been on a what if' basis but the person is serious about defecting.
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