Alistair Darling
Darling led the Better Together campaign to victory at the Scottish independence referendum in SeptemberReuters

Former UK Chancellor Alistair Darling is to step down as an MP at the 2015 general election.

The Labour MP, who represents Edinburgh South West, told the Financial Times that he wanted to leave the House of Commons while he was "relatively young".

Darling, who served under Gordon Brown as Chancellor between 2007 and 2010, led the Better Together campaign to victory (55.3% vs 44.7%) at the Scottish independence referendum in September.

But the Labour heavyweight said he was frustrated that Labour had not capitalised on the victory.

"My frustration is that we actually won," he said. "You can't say it often enough. We made the arguments, we had confidence in ourselves."

His comments come after a shock poll from Ipsos MORI for STV News put the Scottish National Party 29 points ahead of Labour in Scotland.

The survey, which questioned more than 1,000 people in October, found that Labour would only win 23% of the Scottish vote, down from the 42% it secured in 2010, at the 2015 General Election.

The figures came after Johann Lamont resigned as the leader of Scottish Labour.

Darling endorsed Jim Murphy to take over from Lamont, who stepped down from Ed Miliband's shadow cabinet to concentrate on the leadership campaign.

"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," Darling said.

Despite stepping down as an MP next may, Darling said he hoped to be part of a campaign to keep Britain in the European Union as he believes a referendum on the UK's membership is inevitable.

"It's a boil that has to be lanced," he said. "If you sit back and wait till the other lot have taken so much ground then you're on the back foot."

The resignation announcement will be a blow to Miliband as other Labour heavyweights, including the former Foreign Secretary Jack Straw and the Former Home Secretary David Blunkett, have also said they are going to step down in May.