The Labour leadership contest has erupted into a bitter row after the frontrunner’s campaign team were forced to vehemently deny claims they had hacked into party membership data.

Keir Starmer’s team called the allegation, made in official letters from Labour, “utter, utter nonsense”, saying the inquiry only began after they raised concerns about the data use of another candidate, Rebecca Long-Bailey.

Allies of the shadow Brexit secretary said the investigation and the way it was leaked showed that elements in Labour wanted to hamper his chances and help Long-Bailey, who is seen as the preferred candidate of the left of the party which still dominates its day-to-day running.

The dispute erupted as Starmer took time away from the contest because of a family bereavement. His mother-in-law, who had been critically ill, died on Saturday.

Jenny Chapman, the former Darlington MP who lost her seat in December’s election and now chairs Starmer’s campaign, tweeted: “My thoughts and prayers are with Keir Starmer and his family. Keir’s mother-in-law died yesterday following an accident two weeks ago.”

Labour has rejected any bias, saying it is under a legal responsibility to tackle any potential misuse of data, and that it had written to all the remaining candidates reminding them of their obligations.

The row began after Labour officials wrote to two members of Starmer’s team accusing them of effectively hacking into the party’s membership database. The complaint has been referred to the Information Commissioner’s Office.

Within days of the referral being made on Thursday, news of the process was passed to the BBC. The BBC declined to say who had passed on the information.

A spokesman for Starmer’s campaign said: “We categorically reject these nonsensical allegations and are incredibly disappointed that they have been leaked to the media. We are still awaiting the party’s formal response to the serious concerns we and others had about access to Labour party membership data.”

Q&A How does the Labour leadership contest work? Show The first stage of the contest was for potential contenders to get the backing of 22 fellow MPs by 13 January. Five MPs passed this threshold: Keir Starmer (88 nominations), Rebecca Long-Bailey (33), Lisa Nandy (31), Jess Phillips (23) and Emily Thornberry (23). The second stage required each contender to win the support either of 33 constituency Labour parties (CLPs); or of three affiliates, two of which had to be unions, and which between them accounted for at least 5% of the affiliated membership. This had to be achieved before 14 February. Jess Phillips withdrew from the contest on 21 January. Emily Thornberry failed to attract the required number of members. The ballot of members and registered supporters was due to open on 21 February, and closes at noon on 2 April. To be eligible to vote you must have been a Labour member on 20 January, or have applied to have become a £25 registered supporter by 16 January. Corbyn’s successor - Starmer, Long-Bailey or Nandy - will be announced at a special conference in London on 4 April.

Chapman said events began when Long-Bailey’s campaign sent an email to thousands of supporters that included a link to a database of potential backers who could be contacted.

This link appeared to access the Labour party phone bank of contacts, which contravened leadership rules, Chapman told BBC Radio 5 Live. Chapman said the Starmer campaign then alerted the party to what, appeared to possibly be an inadvertent data breach.

“And that was really the end of it, as far as we were concerned. The next thing you know, a couple of people on our campaign receive letters saying: ‘Actually, we think you’ve done something wrong.’ It’s utter, utter nonsense.”

According to the BBC, the letters accuse Starmer’s team of “data-scraping” – in effect hacking membership data to bolster their campaign efforts.

Chapman declined to speculate on why this could have happened or who passed the information to the BBC. “Whoever decided to send these threatening letters to people on the Keir Starmer campaign and then to leak it to the BBC really isn’t doing the Labour party or their preferred candidate any favours here,” she said.

Speaking on the same programme, Lord Falconer, a minister under Tony Blair, said he assumed it was intended to damage Starmer. “It tells us much more about the sense of the left in Labour losing its grip on the party,” he said. “Do you believe Keir Starmer or do you believe that shower that has been around dealing with, for example, allegations of antisemitism. My money is on the Keir Starmer side.”

A Labour spokesman said: “The Labour party takes its legal responsibilities for data protection – and the security and integrity of its data and systems – extremely seriously. We have written to all leadership candidates to remind them of their obligations under the law and to seek assurances that membership data will not be misused.”

It is understood that Jennie Formby, the party’s general secretary, has written to all candidates to remind them about proper use of data, and that so far the party has not uncovered evidence of wrongdoing by Long-Bailey’s team.