The misuse of UK citizens’ personal data was “rife” between Brexit campaign group Leave.EU and other businesses owned by its founder Arron Banks, according to a former Cambridge Analytica employee who worked with Leave.EU, Ukip and Mr Banks’s company Eldon Insurance before the referendum.

Brittany Kaiser, who left Cambridge Analytica (CA) in March after four years, told MPs she believed Leave.EU leadership had combined data from Ukip members, Eldon Insurance and GoSkippy Insurance customers as well as Leave.EU data for analysis by a US university and the targeting of political messages around the EU referendum.

In written evidence provided to the parliamentary fake news inquiry, Ms Kaiser said: “In hindsight, I now think that there is reason to believe that misuse of data was rife amongst the businesses and campaigns of Arron Banks.

“If the personal data of UK citizens who just wanted to buy car insurance was used by GoSkippy and Eldon Insurance for political purposes, as may have been the case, people clearly did not opt in for their data to be used in this way by Leave.EU.

“I have similar concerns about whether Ukip members consented to the use of their data.”

Andy Wigmore, Leave.EU’s communications director, called Ms Kaiser’s statements a “litany of lies” which she invented “to fit the anti-Brexit narrative”.

Appearing before the Select Committee for Digital, Culture, Media and Sport (DCMS), Ms Kaiser said she had delivered “parallel pitches” for Leave.EU, Ukip and Eldon Insurance in October 2015 to “find the different personality types of individuals most likely to engage with political messaging”.

Based on this work, Ms Kaiser said she advised Leave.EU teams on “crafting communications campaigns” in November 2015.

Mr Banks asked CA to “design a strategy” for combining the work for all three organisations, she said, but the submitted proposal was never carried out.

She added: “We were never contracted to do this piece of work, although from other evidence given by other individuals … I do believe that this work was carried out, just not by Cambridge Analytica.”

Neither Ukip nor Leave.EU paid the invoice for the initial work, she said.

Instead, Ms Kaiser said Leave.EU executives created “their own Cambridge Analytica” based on the proposals she submitted.

“Arron Banks and Andy Wigmore have told multiple individuals that they took my proposal and copied it and they created their own Cambridge Analytica, which they called Big Data Dolphins in partnership with the data science department at the University of Mississippi,” she said.

Labour MP Jo Stevens asked Ms Kaiser to clarify whether her proposal included “taking data from UK customers of Eldon Insurance, GoSkippy Insurance, Ukip members … Leave.EU data and that goes over into the United States”.

Ms Kaiser replied: “Yes.”

Asked directly whether Eldon Insurance data was used by Leave.EU, Ms Kaiser replied “that’s what I saw with my own eyes”, describing a call centre at the headquarters of Eldon Insurance in Bristol where employees campaigned for Leave.EU and “every single person that they called was a lead or a customer from Eldon Insurance or GoSkippy”.

Ukip’s former head of communication, Gawain Towler, told the Press Association Ukip did not pay CA for any services because it did not receive them.

He said: “Nobody denies that Ukip met Cambridge Analytica. Nobody denies we asked them to prepare a bid. They prepared a bid, showed us their preparations and we said ‘no thank you’.”

Eldon Insurance did not immediately respond to requests for comment.

The Information Commissioner’s Office (ICO) announced in early April it was investigating “30 organisations, including Facebook” over the use of data and analytics.

Mr Wigmore insisted that Eldon Insurance had not shared any data and that no data had been sent to Mississippi.

He said in a statement: “The fabrications are astonishing. The fact that (committee chairman Damian) Collins and his committee are still refusing to summon Arron Banks demonstrates that they are not interested in the truth. All they care about is spinning fibs to undermine Brexit and the Leave campaign.

“Collins and his fake news inquiry should call Arron Banks to appear before the committee so he has a fair chance to refute these ludicrous lies.”

Former Cambridge Analytica chief executive Alexander Nix was due to give evidence to the committee for a second time on Wednesday April 18 but has declined to appear, citing an ongoing investigation by the ICO.

An ICO spokesperson said: “We have written to Mr Nix to invite him to be interviewed by our investigators. Our investigation is looking at whether criminal and civil offences have been committed under the Data Protection Act.”