The UK's music industry body is set to take the Pirate Party UK to court in a dispute over offering access to banned site The Pirate Bay.
Minor political group Pirate Party UK set up a proxy service to the piracy site ahead of its blocking by the High Court in April.
Leader Loz Kaye refused the British Phonographic Industry's (BPI) request to shut down the proxy last week.
The BPI said court proceedings "are going to be necessary".
The industry body told the BBC its solicitors will writing to the party in due course.
"Despite our efforts to resolve the matter amicably, it is clear that the Pirate Party are determined to continue providing access to the illegal Pirate Bay site," a spokesman said.
"Our solicitors will now be formally writing to the members of the Pirate Party's national executive committee."
In a statement, Mr Kaye said his party was ready to defend itself.
"It is clear that we are facing a significant threat, and we will have to fight it.
"And fight it well, not just for the sake of the Pirate Party, but because of the principles at stake.
"I have always believed that it is not just enough to have principles, you need to act on them too, even if it gets difficult."
In April, the High Court ruled that the UK's major internet service providers (ISPs) must block The Pirate Bay, one of the most popular sites listing links to download illegal copyrighted material.
The ISPs involved in the action have complied, but internet rights campaigners have voiced criticism of the move, suggesting it was ineffective against piracy and a form of censorship.
The Pirate Party UK - not associated with The Pirate Bay - had set up a proxy service which allowed UK internet users to circumvent the block.
After the court-ordered ban came into effect its traffic skyrocketed. According to web metrics firm Alexa, the party's site was ranked 1,943 in the UK prior to the Pirate Bay ban.
The site is now ranked 147 - higher than the likes of Netflix, the Huffington Post and the NHS. Mr Kaye has previously boasted that it sent more than two million hits to the Pirate Bay every day.
On its website, the Pirate Party UK said the BPI was trying to "kill" it.
The BPI responded: "We are asking for only the proxy to be removed and for the Pirate Party not to provide any other access to The Pirate Bay.
"There is no issue with Pirate Party expressing their views on any other section of their website."