MPs who work with their wives may benefit from an extra household salary at public expense, but it comes with complications.

MPs who work with their wives may benefit from an extra household salary at public expense, but it comes with complications.

Sir Menzies Campbell once had to give his wife - noted for being a formidable presence by his side - a formal warning for describing a persistent letter-writer as "a nutter". As his constituency secretary for the past 21 years since he was first elected in North-east Fife, she accidentally sent her own Post-It note to the man, and the LibDem MP had to assure him that his staffer was duly given a severe dressing down.

While Tory leader David Cameron yesterday admitted more than a third of his MPs employ family members and many treat it as a top-up to an eroding MP's salary, there are others who say their spouses provide a loyal public service that tightly constrained salaries cannot buy.

Lady Elspeth Campbell "works Saturday and Sunday and doesn't take overtime or her holiday entitlement," said her husband.

"You could say she's exploited by her employer. She deals with constituency cases and my diary.

"When I was party leader, she had the tricky business of a co-ordinating role with the constituency, and now, if anything, she's working harder because there's a lot more to be done."

At Holyrood, family employment as MSP staff is common, including the mother of Deputy First Minister Nicola Sturgeon, the sister of Tory Ted Brocklebank and the husband of Labour's Elaine Murray.

SNP MSP Alex Neil has employed his wife, Isabella, since becoming an MSP nine years ago. She had previously been his secretary when he was a business consultant.

"We kept that going for obvious reasons, and it works well," he said. "She works hours that no-one else would. You couldn't get anyone else to come in to work at ten to eight and work past seven, at weekends and recesses.

"I've known her to be working for me at 9am on Sunday morning, and on Christmas Day, she handled a constituent's call. You couldn't easily find someone else to do that."

The other side of the argument is that MSPs have voiced worries that their staff have little chance of career development or seniority pay, making it hard to retain good non-family staff. Mr Neil cites two former unrelated Holyrood researchers, who now earn more than three times as much as they did when they worked for him.