Conservatives query rival parties' election expenses
The Conservatives have written to the Electoral Commission over the way rival parties reported their spending on 2015's general election.
MP Charles Walker queried costs relating to "battle buses" for Labour and the Lib Dems and an SNP helicopter.
Several police forces are believed to be investigating whether the Tories illegally reported their costs.
The party has blamed an "administrative error" for not declaring £38,000 of costs from its battle bus tour.
It says that as the battle bus was part of its national campaign, the costs - which related to activists' accommodation - did not need to be reported locally.
But a Channel 4 investigation alleged the operation was focused locally, so should have been declared at constituency level - where strict spending limits apply.
Now Mr Walker has said other parties "have interpreted the rules on election spending in a similar fashion to the Conservative Party".
He cited examples of other parties' spending, asking for the guidance on local and national spending to be clarified.
If the issues were not looked at, he said, "it could give the impression that the commission is not being impartial or, indeed, is behaving in a way that could lead to it being accused of political bias".
He said the Liberal Democrats allegedly used a battle bus to transport activists around the country, including Chippenham, as reported by the Guido Fawkes website.
The Lib Dems said the bus was not used to transport activists but to carry the then leader Nick Clegg, along with his staff and some journalists, and was correctly allocated as a national expense.
A spokesman said this was "entirely different to the series of complaints facing the Conservatives".
He added: "Transport and expenses for these activists should appear in local expenses. That is the substantive issue for investigation, not the buses in which Cameron, Miliband or Nick Clegg travelled the country."
Mr Walker's letter also queried SNP leader Nicola Sturgeon's helicopter travel, saying it had been used to campaign in 12 target constituencies but declared in the national return.
An SNP spokesman said: "Party leader tours are not the same as bussing in activists into marginal seats. We have correctly registered our party leader transport costs as such with the Electoral Commission."
Mr Walker also inquired about Labour's use of battle buses and former deputy leader Harriet Harman's pink van targeting women voters.
The commission said it was considering his letter and would respond in due course.
It said: "The commission's role is to regulate national party spending. Allegations surrounding candidate spending returns are a matter for the police to investigate and there are a number of ongoing investigations.
"The commission has been calling since 2013 for the power to regulate candidate spending."
Labour has previously said its expenditure was "part of a nationally branded tour so the transport costs are rightly national spend".
"Labour's spending is within the law and the rules set out by the Electoral Commission," it added.
Meanwhile, the Electoral Commission has denied its own investigation of Conservative election expenses has been compromised by comments made by one of its staff members on Facebook.
Channel Four News has reported that the Conservatives have formally complained to the watchdog about postings by Louise Edwards, who is head of regulatory compliance and casework.
The posts, written six years ago, are reported to have said that she did not want to live under a Conservative government.
In a statement, an Electoral Commission spokesperson said: "We do not comment on ongoing investigations. However, we are satisfied that the commission is acting fully in accordance with our enforcement policy and with complete impartiality."