A senior Tory MP has written to the elections watchdog to highlight the spending of rival parties on their battle bus campaigns.
The Conservatives face an Electoral Commission probe and a series of police inquiries over allegations that spending on buses transporting activists to campaign in key marginal seats was not properly recorded.
Tory MP Charles Walker has written to the commission urging it to "clarify its guidance" about the rules governing campaign buses and also highlighted SNP leader Nicola Sturgeon's use of a helicopter to travel to battleground seats.
He said that if the commission failed to examine the Labour, Liberal Democrat and SNP campaigns it could give the impression it was "behaving in a way that could lead to it being accused of political bias".
The Conservatives are being investigated over allegations that the cost of its battle bus 2015 campaign should have been recorded as counting towards individual candidates' spending limits, rather than as part of the larger national return.
Broxbourne MP Mr Walker wrote to Electoral Commission chairwoman Jenny Watson saying "a number of political parties have interpreted the rules on election spending in a similar fashion to the Conservative Party".
He urged Ms Watson to look into the issues he raised about the Labour, Liberal Democrat and SNP campaigns "with a view to clarifying the guidance on local and national election spending", warning that if it did not examine the cases "it could give the impression that the commission is not being impartial".
The letter marks an attempt by the Conservatives to fight back after a series of allegations about their election expenditure.
Mr Walker told Ms Watson: "You will no doubt be aware that a number of political parties have interpreted the rules on election spending in a similar fashion to the Conservative Party."
Examples highlighted by Mr Walker were:
:: Some 13 Labour candidates received visits from Harriet Harman's "pink bus" but did not declare this in their local returns, with the cost instead included in the national return.
:: The Liberal Democrats reportedly used an election battle bus to transport activists to constituencies including Chippenham, which was not included in candidate Duncan Hames' return. Mr Walker said: "I suspect he was advised by his party that such a declaration was not necessary."
:: The "Labour Express" campaign saw activists transported across the country, including Ealing Central and Acton, where candidate Rupa Huq did not record it as part of her spending return "again probably because she was advised that she did not need to by her party", according to Mr Walker.
:: Buses with student activists visited 17 Labour marginals, the costs including accommodation "appear to have been declared on Labour's national return".
:: Ms Sturgeon "used a helicopter to campaign for SNP candidates in 12 target constituencies - at a cost of £35,000", Mr Walker said.
Mr Walker said the commission had previously noted that the legal position in relation to party spending focused on constituencies "would benefit from clarification".
He told Ms Watson: "I would be grateful if the Electoral Commission could look into the issues that I have raised, regardless of the party involved, with a view to clarifying the guidance on local and national electoral spending.
"If the Electoral Commission does not look at these issues, 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."