The far-right British National Party says it is considering legal action after its bank accounts were frozen by Barclays.
The bank's move is thought to be in response to a BBC documentary, which showed BNP members confessing to racially motivated crimes.
BNP chairman Nick Griffin claimed the action breached European human rights legislation.
"We have the right to hold political opinions and to impart them," he said.
The bank has not confirmed the decision but the BNP's John Walker said he had received a call from the bank informing him that the accounts are to be closed.
Mr Walker said that he was unaware whether any other banks that hold party funds would follow suit.
But Mr Griffin said the party may find it difficult to open accounts with other banks.
He described the move by Barclays as "absolutely scandalous" and an "attempt to ban it [the party] by the back door".
He said a member of the party's treasury department was informed about the freezing of their accounts on Friday afternoon.
The legal position of the account closures will be studied, he added.
"If we can, we will take them [Barclays] to the cleaners.
"We don't want to take them to the cleaners, all we want is the democratic right to access back accounts."
He said: "Barclays may find themselves up against European human rights legislation."
Mr Griffin said the accounts contained thousands of pounds - "enough money to run a small but effective political party".
The party must have a bank account in Britain to comply with electoral commission regulations.
A Barclays spokeswoman said the company would not comment because of client confidentiality.
However, a bank official said Barclays would consider whether there was a "reputational risk" when it was deciding whether to open or close an account.
In The Secret Agent documentary, footage recorded at a meeting in Keighley shows Mr Griffin saying Islam was a "vicious wicked faith" and warning the audience to "stand up" to Muslims.
In an interview on BBC Two's Newsnight programme following Thursday's broadcast of the documentary, Mr Griffin refused to say sorry for his comments and added the "Islamification" of the West had partly happened by rape.
However, he did apologise for comments made by other BNP activists shown on the documentary confessing to race crimes. Three of them have been expelled from the party, Mr Griffin said.
Reporter Jason Gwynne spent six months infiltrating the BNP's West Yorkshire branch with the help of a former local organiser.
West Yorkshire Police and the Crown Prosecution Service, in a joint statement on Friday, said officers were collecting tapes from the programme makers to review.
"The BBC programme (Secret Agent) broadcast last night raises a number of issues which warrant further investigation.
"Working with our colleagues in the Crown Prosecution Service we will be reviewing the material to identify what, if any, information of evidential value it contains and decide on the appropriate action."