Alistair Darling, the former ­chancellor and leader of Better Together who is to stand down as an MP, has criticised his Labour Party for failing to capitalise on victory in the independence referendum.

Mr Darling, 60, said last night: "My frustration is that we actually won. You can't say it often enough."

He warned that Labour in Scotland had too often sat back instead of engaging with the political fight.

But he insisted his departure was not evidence of further disarray within Scottish Labour after the resignation 10 days ago of Scottish Labour leader Johann Lamont.

Mr Darling, who will not contest his Edinburgh South West seat at the General Election in May, also warned David Cameron his push to restrict the voting rights of Scottish MPs could create a financial crisis.

He attacked the SNP, accusing the party of creating a state of "neverendum".

He advised the Prime Minister to "think long and hard" about the repercussions of his proposals for "English votes for English laws".

He said that, if an opposition party had a majority among English MPs, it could vote down the Westminster Government's Budget. Even the threat of such an outcome could lead to market uncertainty about the state of the economy, potentially pushing up borrowing rates, he warned.

He compared the situation to that which can paralyse the United States.

He said: "The president of the United States has to go down on bended knee and to plead with Congress to get his budget through.

"The Tories need to think long and hard about what they want. This is a huge problem."

Mr Darling gave his support to his fellow former cabinet colleague Jim Murphy as the right person to lead Scottish Labour.

"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 said.

Mr Darling said he hoped to play a role in keeping the UK in the European Union. He wanted to do other things while he was still "relatively young".

He suggested the independence referendum had shown that "if you sit back and wait till the other lot have taken so much ground you are on the back foot, you pay a heavy price".

Mr Darling's decision not to stand in May marks the end of a career as an MP lasting almost three decades.

He was first elected as a Labour MP in 1987 after a career as a lawyer.

He said his announcement was necessary to allow the party to prepare to fight the seat in a General Election in six months time. He told Labour leader Ed Miliband of his decision at the end of October.

The MP has a majority of almost 8,500 votes in his Edinburgh South West constituency.

However, a poll last week suggested that Labour could lose almost all of its seats north of the Border, including that of Mr Darling, to the SNP.

Mr Darling was one of just three Cabinet ministers to serve continuously throughout the last Labour Government from 1997 to 2010.

As chancellor in 2007 he was at the centre of the crisis over the collapse of Northern Rock and the subsequent global banking meltdown.

Better Together, the cross-party campaign he led from 2012, was criticised by some for being too negative and failing to make a positive case for the Union.

But Mr Darling was praised for his performance the first TV debate, and in the second, outgoing First Minister Alex Salmond was judged to have fared much better than the leader of the No campaign.

Former foreign secretary David Miliband paid tribute to Mr Darling as "a gentle giant of politics: high on integrity, low on ego".

Mr Miliband said Mr Darling was a man of "values, decency and kindness" who had distinguished himself as an "extraordinary public servant".