Douglas Carswell is accused of passing data to Tories to help them win South Thanet in 2015, according to Ukip donor Arron Banks

This article is more than 4 years old

This article is more than 4 years old

Senior members of Ukip have accused the party’s only MP of helping the Conservatives defeat Nigel Farage in South Thanet in the general election last year, according to Ukip’s main donor, Arron Banks.

Farage, then the leader of Ukip, was beaten by the Tory candidate, Craig Mackinlay, after a controversial campaign in the Kent constituency.

Banks’s company has written to Kent police with the allegation that Douglas Carswell, the Ukip MP for Clacton, helped the Tory campaign retain the seat. It details allegations that Carswell downloaded Ukip data for South Thanet and passed it to the Conservatives, enabling them to do “push polling” of key voters.

Push polling is when an apparently unbiased telephone survey spreads negative rumours about a candidate.

Carswell defected to Ukip from the Tories in 2014 but has had a fraught relationship with both Banks and Farage.

According to the letter, Carswell was granted access to the Ukip database but then only accessed the South Thanet data.

A letter sent to the police by Precision Risk & Intelligence, where Banks is chief executive, claims that “we have evidence of excessive spending by the Conservatives and secretive dealings between them and a senior Ukip representative to collude against Mr Farage”.

Arron Banks (@Arron_banks) Letter sent Kent police - the same call centre push polled Rochester. Dirty politics..... pic.twitter.com/SPxuQKaqEp

Responding to the accusations, Carswell said: “There is no basis in these claims whatsoever. We should just be relieved that those responsible for the disastrous campaign in South Thanet were not responsible for the successful referendum campaign.”

It may be that any unauthorised use of the Ukip database would be a breach of data protection laws.

The letter also claims that the information was passed to a call centre in New Malden, Surrey, and was then used to target voters in South Thanet. The call centre in Surrey has close links to the Conservative party.

Kent police are investigating allegations of improper election spending by the Tories in South Thanet, a highly marginal seat at last year’s election. They were recently granted a further 12 months to investigate electoral spending in the constituency after a judge concluded that the inquiry could lead to the result “being declared void”.

It is one of several seats under scrutiny following claims of illegal overspending. There are strict spending limits on both national and local campaigns, and police are investigating whether limits were breached.

Electoral law allows candidates to spend between £10,000 and £16,000 depending on the size of the electorate. In South Thanet, Mackinlay said his campaign spent just below the limit, which was £15,016.

However, questions have been raised about the total spending in the seat, with claims that money spent in South Thanet was listed under national spending. The Conservatives have defended their election spending, saying it was within the law.

As part of the Panama Papers investigation earlier this year, the Guardian revealed that Banks was a shareholder of the holding company of Precision Risk & Intelligence. The company, PRI Holdings, was run in the British Virgin Islands by the Panama law firm Mossack Fonseca.