The commission said it would consider what action to take
A row has broken out over an inquiry by the Electoral Commission into the the SNP's financial affairs in Angus.
It stems from a complaint from the local Conservative association over the distribution of leaflets.
In a letter, the commission said it was "reasonable" for the Tories to believe a breach of the law may have occurred.
The SNP said if the commission had "bothered to approach the party with basic questions" it would have realised all of its accounts were in order.
A spokeswoman said the party was seeking a full apology from the Electoral Commission over this issue.
The accusations centre on the lack of accounts submitted to the commission by the SNP in Angus.
'Not definitive evidence'
If income or spending is higher than £25,000 constituencies have to submit detailed accounts.
However, Alberto Costa, the Conservative parliamentary candidate for Angus, said the SNP had tried to circumvent the law by setting up four "accounting units" in Angus.
The commission had previously decided not to carry out a full investigation, but agreed to review the evidence after being sent SNP leaflets supporting MPs, SMPs and councillors that all stated they were "published and promoted by Angus SNP".
In a letter to Mr Costa, the commission said: "Whilst not being definitive evidence that the Angus Accounting Unit's accounts would be over the reporting threshold, this does suggest that the Angus accounting unit has more than a minimal turnover."
It added: "Following an initial assessment of the information, we concluded that it was reasonable to believe that a breach of [the Political Parties, Elections and Referendums Act] may have occurred and that further inquiries should be made in order to establish the facts and to decide what, if any, action may be appropriate."
SNP MP Mike Weir said the Conservatives were engaged in "mudslinging" and the accusations were "ludicrous" and "utter nonsense".
A spokeswoman for the party added: "The commission have advised that they are not conducting a formal investigation into the unsubstantiated allegations made by the Tories but, in line with their allegation assessment procedures, are simply looking to establish the actual facts.
"We will cooperate fully with the commission and provide any information requested by them.
"Our local organisations in Angus submit their accounts in line with Electoral Commission requirements."
The Angus constituency includes Mr Weir's Westminster constituency, as well as John Swinney's and Andrew Welsh's Holyrood constituencies.