The commission, which is the independent party finance watchdog, said it had began the probe into the party's 2008 statement of accounts.
This follows concerns raised about its "adequacy" by the independent auditor's report that accompanied it.
But the fact that an investigation had been launched did not mean there should be assumptions made over any alleged breaches, the commission stressed.
In January this year the commission began a review following the concerns raised, and this has now developed into the investigation announced today.
It follows comments by the registered auditors, Silver & Co, that the financial statements submitted did not "give a true and fair view of the state of the party's affairs at December 31, 2008".
The auditors went on: "In our opinion it cannot be said that the accounts comply with the requirements of the Political Parties, Elections and Referendums Act 2000, as adequate records have not been made available."
A statement released by the commission said: "In January 2010 the Electoral Commission, the independent party finance watchdog, began a case review following concerns raised in the independent auditor's opinion about the adequacy of the 2008 statement of accounts of the British National Party.
"The case has now become an investigation.
"However, it is important to note - particularly during an election period - that no conclusion has been reached and therefore no assumption should be made as to whether a breach of the rules has occurred."
The Independant
