Former chancellor Alistair Darling has announced that he is to stand down as an MP at the next general election.
The 60-year-old MP said that he wanted to step aside while he was still "relatively young".
Darling led the Better Together campaign to keep Scotland in the United Kingdom in the referendum, but has said he feels "frustrated" that his party hasn't capitalised on the campaign's successful 'no' vote result.
Alistair Darling will not stand as an MP in 2015
He told the Financial Times he was exasperated that the Labour party in Scotland has not used the Better Together campaign as a springboard to boost support.
Support for Labour in Scotland has collapsed since the vote in September, and recent opinion polls suggest that the party may lose the vast majority of its seats and be wiped out. Meanwhile there has been a surge in support for the Scottish National Party.
"My frustration is that we actually won," Darling said. "You can't say it often enough. We made the arguments, we had confidence in ourselves."
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In a parting shot, Darling also threw his weight behind Jim Murphy to become Labour's new leader in Scotland.
"Jim has the enthusiasm, the energy and above all he's a fighter. For too long we have sat back when we needed to fight," he told the FT.
Darling, the MP for Edinburgh South West, was one of only three cabinet ministers to serve continuously throughout the years of the last Labour government from 1997 to 2010.
When Gordon Brown became prime minister in 2007, he was given the post of chancellor and swiftly found himself at the eye of the storm with the collapse of Northern Rock and the subsequent global banking crisis.
His leadership of the cross-party Better Together campaign from 2012 was criticised in some quarters for being too negative, focusing relentlessly on the economic risks of Scotland breaking away from the United Kingdom.
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He surprised some, however, when he got the better of SNP leader Alex Salmond in the first of their two televised debates in the run-up to the referendum vote, although the Scotland First Minister was generally perceived to have got the better of the return match.
Former foreign secretary David Miliband paid tribute to Mr Darling as "a gentle giant of politics: high on integrity, low on ego".
Times columnist and Conservative activist Tim Montgomerie tweeted that Darling is "one of the grown ups in politics", while others expressed their admiration for the MP's work leading the Better Together campaign.
When Alistair Darling gets up and speaks in the Commons, all sides of the House of Commons shut up and listen. He'll be greatly missed.
— Chuka Umunna (@ChukaUmunna) November 2, 2014
But others speculated that Darling's decision was a move to jump before he was pushed, after seeing the recent opinion polls suggesting Labour would lost the vast majority of its Scottish seats in the 2015 election.
Half (49%) of Britons in an Ipos Mori poll also thought the party should remove Ed Miliband as their leader before the election.