ANALYSIS: Questions have been raised over David Marshall's office costs, writes Douglas Fraser

A spotlight on the expense claims of Labour's old guard of MPs is not what the party wanted in the week of a crucial by-election.

But that's what a Labour-supporting newspaper delivered yesterday. It revealed that David Marshall, whose sudden resignation as MP for Glasgow East sparked tomorrow's contest, has been claiming for the costs of running his constituency office from his home while his daughter, Christina, has been running two property rental businesses from the same address.

There is no proof that the same office is used for both, as it is larger than the average home in Glasgow's east end. So this is not clear evidence of public money being misused. But for Labour, the coincidence of a home office generously funded by the public, with a private business at the same address, is a badly-timed reminder of an old guard of MPs whose expense-claiming practices have come under a harsh glare in recent months.

It also returns to questions about why Mr Marshall had to resign so abruptly at the end of June. The party said it was down to medical advice, including a second opinion, advising him he should step down immediately. But however gravely ill he may be, he would not have been the first MP to remain in post for several months or sometimes years, without ever appearing in the Commons. Commons whips would surely have preferred Mr Marshall to lie low in Glasgow than to force a by-election in the wake of Labour's heavy defeat in Crewe and Nantwich and lost deposit in Henley.

A notable omission from the Labour campaign in Glasgow has been any endorsement from Mr Marshall. Nor has the party helped explain his family business arrangements and expense claims. David Marshall has been airbrushed. The campaign has focused on candidate Margaret Curran, to underline this is about Glasgow East's future rather than the Labour's past.

The key factor raising eyebrows about the timing of Mr Marshall's resignation is the publication, in the next few weeks, of details of MPs employing their own family members. October is scheduled to see publication of claims under the equally controversial second-homes allowance, with media and public astonishment at the extent to which London flats are kitted out at public expense.

For Mr Marshall, information already in the public domain shows a "C Marshall" - both his wife and his daughter are called Christina - earned at least £75,000 over recent years. He charged the Commons authorities a reported £17,000 per year for the costs of running his constituency office out of his Glasgow home.

It may all have been within the rules, but these lax rules have left MPs struggling to explain questionable practices in recent years. While stories of expenses scams are part of Westminster folklore, the issue which has already derailed several Holyrood careers is now threatening a Westminster train wreck.

Earlier this year, Tory leader David Cameron conceded MPs had sought to make up for what they saw as the falling real value of their salaries by putting family members on the public payroll. He recognised his party had to move swiftly to avoid the controversy stalling his progress in the polls.

The Conservative MP whose flagrant abuse of the rules started the juggernaut of expenses controversy rolling at Westminster was Derek Conway, employing his son while he was a full-time student in Newcastle. And the party's chairwoman, Caroline Spelman, continues to undermine Tory attempts to pin this controversy on Labour, with an investigation of claims that she used Commons expenses to employ a nanny. Needless to say, her party role involves overseeing probity in Tory expense claims.