The British National Party is under investigation by the European Union and the Metropolitan Police for alleged fraud and breaches of electoral law.
The dual investigations come as a former BNP administrator told the BBC's Panorama programme that she was instructed to falsify invoices.
Those invoices were then submitted by the BNP to the Electoral Commission.
The BNP has strongly denied any suggestion of wrongdoing.
The allegations come as the party struggles with debts run up during the 2010 general election campaign.
Internal party documents seen by Panorama reveal that 12 months ago the BNP owed creditors more than £570,000. Party chairman Nick Griffin recently said the party now owes just £52,000.
Former party worker Marion Thomas said after the 2010 general election she was instructed by the party's treasurer, Clive Jefferson, to alter invoices and in at least one case stamp an outstanding invoice as "paid".
The invoices were submitted to the Electoral Commission and had been altered, Mrs Thomas said, in order for it to appear that the BNP had complied with the law on election spending.
Asked how she felt about doing this, Mrs Thomas said: "I made my objections known."
She added: "You can't do that, you cannot do that. That is fraud."
Mr Jefferson told the programme that Mrs Thomas' allegations are "untrue".
Mrs Thomas, who now works for Britain First, a rival political organisation, has since been interviewed about her claims by detectives from the Metropolitan Police who are investigating alleged breaches of electoral law by the BNP.
That investigation began after Richard Barnbrook, who used to be the BNP member of the London Assembly and Mr Griffin's 2010 election agent, went to the High Court to say that he had submitted printing invoices totalling nearly £10,000 as paid when they too were outstanding.
Mr Griffin also signed those returns. Both he and Mr Barnbrook, who has since been expelled from the party and now sits as an independent in London, have said they acted in good faith, believing the bills had indeed been paid.
The High Court judge has referred the case the Director of Public Prosecutions and the Metropolitan Police were notified.
Another former party worker, Alistair Barbour, was recruited to Mr Griffin's European staff after he and one other BNP candidate were elected members of the European Parliament in 2009.
Mr Barbour was hired to work on European Parliament business and was to be paid out of the £260,000 pot of EU money that each MEP has available to them to pay for staff and expenses.
He told the programme that some money intended for MEP business was diverted to help bolster the party itself.
"Europe was the big cash cow you know, 'let's get our noses in the trough and see what we can get out and... see what we can fund the party with,'" he said of the approach to the MEP funds.
He added: "This is what it was all about, party work and just trying to figure out what expenses we could get out of the European Union."
Other party insiders have told the programme that at one point electricity from Nick Griffin's European constituency headquarters on an industrial estate in rural Cumbria was siphoned to the unit next door which served as the BNP's national headquarters.
When the European Parliament's fraud unit, OLAF, travelled to Cumbria five months later to investigate the allegations they found no evidence of an election scam but Panorama understands that they continue to investigate other allegations of misuse of European money by the BNP.
The BNP has denied using money from the European Union to fund national party work.
Panorama: BNP - The Fraud Exposed, BBC One, Monday, 10 October at 2030BST and then available in the UK on the BBC iPlayer.