Former Prime Minister Gordon Brown will announce that he is to quit his job as an MP at the next general election.
After 32 years in the Commons, the Labour MP of Kirkcaldy and Cowdenbeath is expected to make a formal announcement before Christmas.
"Gordon always holds a function for his party members at Christmas and if I were a betting man, I would bet that he will make an announcement then", the source told Sky News.
Brown is holding a Christmas party for local councillors and party officials in his constituency on 1 December and is tipped to make the announcement then.
His decision to step down as an MP has been widely expected, though it is understood to have been delayed thanks to his starring role in the recent Scottish referendum campaign.
"He wants to go out on a high after effectively salvaging the campaign to keep the UK together in September", an ally told the Sunday Mirror.
The source also revealed that after his resignation from politics in May 2015, Brown will "focus on his charity work".
Since losing the 2010 general election to David Cameron, Brown, now 63, has become UN Special Envoy for Global Education.
He has been criticised by political opponents for his rare appearances in the Commons, but made a dramatic comeback in the Scottish referendum battle when the 'No' campaign appeared to be heading for defeat.
His barnstorming speech in Glasgow's Maryhill on the eve of the referendum was widely acknowledged as a personal triumph and a decisive turning point in the eventual outcome of the referendum.
Mr Brown's seat, Kirkcaldy and Cowdenbeath, is one of Labour's safest in the UK, with a majority of more than 23,000.
Sky News reported on Sunday that the former PM would like to see an all-women shortlist for the selection and Brown's colleagues are tipping his former speechwriter, Kirsty McNeill, to succeed him.