A data rights group has threatened legal action against the Conservatives, Labour and the Liberal Democrats over the parties’ use of personal data ahead of Thursday’s election.

A pre-action letter has been sent by Open Rights Group on behalf of three individuals who had requested their data from the parties and were dissatisfied with their responses. It includes an “urgent notice” for all three parties to stop processing their data immediately.

The letter cites breaches under the Data Protection Act 2018, saying the “imminence of the election” and “heightened processing activities” necessitated all parties to stop processing immediately. It gave the parties a deadline of 6pm on Friday 6 December to comply. When contacted by the Guardian, none of the parties gave any indication that they intended to do so.

Pascal Crowe, Open Rights Group’s data and democracy project officer, said abuse of personal data was a “systemic” issue in UK politics and there were serious concerns about the legality of the parties’ practices.

Data subject access requests to the Labour party had returned “unintelligible” data. The Conservatives appeared to have been using personal names and address to guess the age of one of the individuals, and the Liberal Democrats had failed to provide the sources of third-party data, he said.

“We are concerned about the lawfulness of these activities and have put these concerns to the parties.” He added that the information that three individuals had obtained from the parties was “baffling”.

“We often don’t recognise our profiles. They are not even profiling accurately.”

Ravi Naik, a data rights lawyer who previously acted for American David Carroll, who sued Cambridge Analytica for data abuse, and is acting for the three individuals on behalf of the Open Rights Group, said there were issues with all three parties.

“None of the parties have provided any information about how those profiles are being used. We know they’re working with outside companies but they’ve listed no political consultancies. In this day and age, we know how important data is to political parties, but they’ve provided almost no information or insight into what they are doing with this personal material.

“People need to know what data these parties have, where they are getting it from, and what they are doing to it. This is their legal right. But we are days away from this pivotal election and we have no idea.”

Naik said that the Open Rights Group was considering “all options”. This included the possibility of a high court injunction to prevent further data processing.

The Conservatives said: “We comply full with all obligations. Where individuals make requests to us regarding their data, we will always action these.”

A spokesman for Labour said: “[We] fully comply with the law and regulations for digital campaigning.”

A spokesman for the Liberal Democrats said: “[We] received a request yesterday from a firm of lawyers acting for three individuals making a request in regards to the processing of their data and related questions. The Liberal Democrats take all such requests very seriously. As with other such requests, we will answer these, as required by law.”