The Chancellor ended his 25-year membership of the Faculty of Advocates a week before Christmas.

Mr Darling, a respected lawyer prior to entering politics, gave no public explanation. However, the Sunday Herald can reveal that the Chancellor’s exit followed a complaint to the Faculty by a member of the public about his House of Commons expenses for a second home.

During the summer scandal over MPs’ expenses, it emerged that Mr Darling had “flipped” his second home four times in four years and received tax advice paid for with public funds.

In addition to a grace-and-favour flat in Downing Street, Mr Darling claimed accommodation costs against a second flat in London and his family home in Edinburgh.

Vince Cable, the Liberal Democrat Treasury spokesman, called at the time for Mr Darling to be sacked as Chancellor after being “caught with his fingers in the till”.

Mr Darling ultimately repaid around £700 of expenses claimed for service charges at his south London flat after admitting making a “mistake”.

The 470-year-old Faculty of Advocates regulates the activity of around 700 members, both practising and non-practising, and its code of conduct states that: “An Advocate owes a duty not to bring the Faculty into disrepute.”

The Faculry rule covers activities in and out of court.

In 1999, the Faculty fined QC Donald Findlay, probably Scotland’s best-known advocate, £3500 for “professional misconduct” after he was filmed singing sectarian songs at Rangers FC Social Club following an Old Firm game. It is understood a complaint was made late last year to the Faculty about Mr Darling’s Additional Costs Allowance, under which MPs can claim the cost of a second home to enable them to perform their parliamentary duties.

His spokeswoman described Darling’s exit as a “coincidence”. However, a legal source said: “There was a complaint lodged by a member of the public about him flipping his house in Edinburgh.

“That automatically activated the disciplinary machinery. He’s learned about it and then decided he didn’t want the bad press and resigned.

“Once an advocate has resigned they are not capable of being disciplined.

“The spin from Number 11 Downing Street is that it’s because he hasn’t practised for a long time, but it was to avoid the embarrassment of the flipping scandal.”

Mr Darling studied law at Aberdeen University in the 1970s then worked full-time as a solicitor in Edinburgh for five years before becoming a Lothian Regional Councillor in 1982.

In 1984 he qualified as an Advocate, allowing him to appear in the High Court. He was elected to parliament in 1987 and now represents Edinburgh South West.

He is defending a majority of just 7242 over the Conservatives, and his seat is among the top 10 targets for the Tories at the election.

Jason Rust, the Conservative candidate in the constituency, said: “We all have a duty to restore trust in politics and in politicians and that means being open and transparent with the voters.

“Alistair Darling should follow the example of Conservative MPs and call a public meeting in his constituency to explain his actions and expenses claims.”

The Faculty confirmed Mr Darling’s resignation but refused to comment on the reasons.

A spokesman said: “It is not the Faculty’s policy to discuss with a third party whether or not a member of Faculty is the subject of a complaint.”

Mr Darling’s spokeswoman said: “There was absolutely nothing wrong in what he did about his houses.

“The disciplinary proceedings would not have been awkward and it is ridiculous to suggest he resigned because of this.”