The Labour party has formally reported two members of Sir Keir Starmer's team to the Information Commissioner, according to the BBC, with one of them being his compliance official.
It is believed the concerns were passed to the Information Commissioner's Office on Thursday, however the leadership front-runner and his team have denied the claims.
Last night, supporters of Sir Keir suggested they were now victims of a politically motivated effort to damage him and his campaign.
The revelation that his campaign had been reported to the Information Commissioner came as it was announced Sir Keir's mother-in-law died yesterday after an accident. As a result he has withdrawn from campaign events.
The move by the party to report Sir Keir's staff also came just days after it emerged the rival leadership campaign for Rebecca Long-Bailey, who is backed by the Jeremy Corbyn supporting campaign group Momentum, had also potentially breached the membership database.
It is understood Long-Bailey's team had shared with its volunteers links to Labour’s official phone-banking system called "Dialogue". The move allowed them to, in theory, contact and lobby any of the party’s 500,000 members.
Ms Long-Bailey's campaign had said there was no attempt to use the system.
The Labour party has said it will investigate whether any data rules had been broken by the Long-Bailey campaign. However the party has taken stronger action over the alleged breach by Sir Keir Starmer's team, leading to accusations of political motivation.
According to the BBC the Starmer team members are accused of "data-scraping" - unlawfully hacking information from the membership database to target support.
Access to members data - crucial to any campaign in garnering support in the leadership and deputy leader elections - is tightly restricted until after February 14, under rules approved by Labour’s ruling National Executive Committee (NEC) earlier this month.
The Information Commissioner's Office (ICO) is the UK's independent body set up to uphold information rights, and has confirmed it has received a report of a membership database breach, and would make inquiries.
Sir Keir has written to the party flatly denied any wrongdoing by his team members. He insisted they were investigating a means of penetrating the Dialogue database with no intention to use it.
A spokesman for Sir Keir'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."
Labour - which appears to have now suspended its phone bank website - has also faced criticism itself for failing to adequately protect the data of its rank and file members across the country.
A Labour Party 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."