Labour is facing accusations of dirty tricks in the leadership contest after an email was sent by a member of party staff to local members advertising a Rebecca Long-Bailey campaign event, in a possible breach of party rules.

BuzzFeed News understands that Labour will investigate any alleged breach of the party’s code of conduct. Staff who have not taken leave to work on leadership campaigns are expected to remain neutral throughout the contest, and rival campaigns have also alleged that the email to members breached data protection laws.

There is no suggestion that Rebecca Long-Bailey or her campaign team were aware of or authorised the email to members in Broxtowe in late January promoting a “listening event” in the East Midlands constituency with Long-Bailey.

It was sent in the name of general election candidate Greg Marshall, who said he was “proud” that the constituency party had chosen to nominate Long-Bailey in the contest.

However, the email was sent out using software called Organise, which allows only authorised accounts, including constituency party officers and Labour staff, to contact members. BuzzFeed News has seen evidence showing that it was sent from the account of one of the party’s full-time community organisers working in the East Midlands.

It raises serious questions over fair play and the use of data in the campaign, as the opportunity to message party members using Organise appears not to have been extended to other candidates. In a statement to BuzzFeed News, Marshall took full responsibility, saying he had asked the staff member to send the email without realising that it was against the rules.

He said: “I asked a staff member to send an email in my name to the CLP [Constituency Labour Party] because the activist who normally sends emails to the CLP wasn’t around that day. This was after our CLP had already held a selection meeting and formally declared our nomination.

“I thought that sending this email was within process and I would’ve afforded the same communication for any candidate who had got in touch. I’m mortified that I landed a staff member in trouble. I sincerely apologise for this genuine mistake."

One Labour source suggested that those involved in sending the email should be suspended while a full investigation into the potential data breach is carried out and that the matter should be referred to the Information Commissioner’s Office. Another said that staff sending emails on behalf of leadership candidates could constitute misconduct.

They told BuzzFeed News: “This appears to show that a member of Labour staff used party data and systems to promote a leadership campaign on behalf of somebody with no legal right to the data. The party must investigate whether there’s been misconduct.”

One insider from a rival leadership campaign said: “Using official Labour data is not just against the rules, it's against the law.”

A member of another leadership campaign told BuzzFeed News: “Party members expect and deserve better than this. The general secretary should stamp out this undemocratic behaviour once and for all.”

Community organisers recently came under the spotlight after a report on their role in the election was described as “propaganda” by party insiders. The controversial staff roles, brought in under Jeremy Corbyn's leadership, have cost the party an estimated £3 million over two years.

A Labour Party spokesperson said: "We don't comment on staffing matters."