An investigation into Nigel Farage's party has been launched by the Information Commissioner's Office.
The Brexit Party is being investigated following complaints it failed to hand over the personal data it holds on voters, Sky News can reveal.
The Information Commissioner's Office (ICO) launched the investigation in response to complaints the Brexit Party had failed to answer requests for data.
Under data protection law, anyone can ask an organisation for a copy of their personal information, a process known as a Subject Access Request.
Unless the request is especially complicated, they are entitled to a response within a month.
A Brexit Party spokesperson told Sky News the majority of Subject Access Requests dated back to the European elections in May.
"During the European elections, there was a coordinated attempt by campaigners to flood The Brexit Party with Subject Access Requests," the spokesperson said.
"All political parties are allowed access to the electoral register so they can send literature to voters. However, inaccurate claims circulated on social media, claiming we had acquired people's addresses improperly, leading to the written requests asking for access to information."
Using Twitter's search function, Sky News found many pro-EU accounts calling for people to file Subject Access Requests to the The Brexit Party in order to find out why they had been sent a party leaflet during the European elections.
Most of the tweets describe wanting to know "how they got your personal details & what they're going to do with them?"
However, some did suggest Subject Access Requests could be used to interfere with the Brexit Party's work.
"European GDPR rules can be weaponised to stop political parties using our data to manipulate us," wrote one on 8 May.
"If a thousand people demanded to see all their data, any organisation would be very busy, to say the least..."
Under data protection law, an organisation does not have to respond to requests that are "manifestly unfounded or excessive".
Sky News understands the data watchdog has given the Brexit Party a deadline of 22 November to answer the requests for data.
The Brexit Party spokesperson said: "We have responded to the vast majority of letters. Around 0.2% are currently being dealt with and we will meet the deadline agreed with the ICO.
"The Brexit Party follows all regulations and works to meet the highest standards."
The ICO told Sky News it would not comment on the investigation.
"As a public body the ICO has to consider its responsibilities during the pre-election period," it said. "Our regulatory work continues as usual but we will not be commenting publicly on every issue raised during the general election.
"We will however, be closely monitoring how personal data is being used during political campaigning and making sure that all parties and campaigns are aware of their responsibilities under data protection and direct marketing laws."
According to the ICO's guidance, the watchdog "cannot punish an organisation for breaking the law (apart from in the most serious cases)."
Instead, it "can give them advice and ask them to solve the problem."
This is not the first time a political party has failed to respond to Subject Access Requests. In August, Sky News reported that the vast majority of complaints to the ICO about political parties' use of data were directed at Labour, although this data did not include the Brexit Party.
Most of the complaints against Labour concerned its failure to respond to Subject Access Requests.
The ability to access personal data is regarded as a foundational data protection right, as it acts as a "gateway" for other rights, including the right to rectification and the right to erasure.
Pascal Crowe, Data and Democracy Project Officer at campaign group Open Rights Group, told Sky News the investigation showed "the increasing importance of data rights in the political sphere".
He said: "Given the controversies of recent years, all political parties should equip themselves to deal with Subject Access Requests at scale. Without this capacity they can expect to see more ICO investigations and fines."
Source: Sky News