Lisa Nandy has written to Labour’s general secretary, Jennie Formby, asking for urgent reassurance about the conduct of the party’s leadership contest, amid growing controversy over why many party members have yet to receive their ballot papers.

The MP for Wigan, who has reached the final stage of the contest along with Keir Starmer and Rebecca Long-Bailey, is concerned that many of those affected by delays are among the 100,000 new members who have joined since Labour’s general election defeat in December.

According to senior party figures, the new members – who, polling suggests, would be more likely to vote for Starmer or Nandy than Long-Bailey, who is backed by the Corbynite left – are being subject to extra verification processes, including checking their addresses against the electoral register. Such checks, it is believed, are not being carried out on longer-established members.

The delays, however, have also hit many members who joined before Jeremy Corbyn became leader in 2015. The former lord chancellor Charlie Falconer, who on Saturday had still not received his ballot papers, said serious questions had to be asked: “The number of people complaining about not receiving their ballot papers gives me real anxiety that there is a high level of incompetence here – or perhaps something worse.”

'These are people less likely to back a Corbyn continuity candidate'

One source close to one of the leadership campaigns said: “We think this is part cock-up, part conspiracy. It is very striking that a very high proportion of those who have not got their papers are either people who joined before 2015, so before Corbyn was leader, or since the general election – so these are people less likely to back a Corbyn continuity candidate.”

The source added: “Many of those who joined after the election were encouraged to do so by [former leadership candidate] Jess Phillips before she pulled out. After she did so, they are by definition likely to have switched to Keir or Lisa.”

Former deputy leader Dame Margaret Beckett, who sits on Labour’s national executive committee (NEC), said she was mystified by the delays: “I know of people both in my family and my constituency who have not had their ballot papers – and this is inexplicable, in that some of these people have been party members for more than 50 years.”

In her letter to Formby – the most senior official in the party – Nandy has demanded answers to a series of questions, including how many members who applied to join since the election in December have not yet passed the electoral register check.

She also wants to know if the same checks have been carried out on people who were already members before the election, and if not why not; and what the position will be for members who are under 18 and who as a result will not be on the electoral register.

In addition, she wants to know why the party considers it “appropriate” to put new “barriers in front of new members joining the party in this election, but not in previous contests” and has asked Formby if the NEC was consulted on the new system.

The ballot opened on 21 February, and closes on 2 April.

It is understood that the full NEC held only a brief discussion of how to manage the leadership contest in January. It is understood that issues relating to membership of the party were placed under the control of Corbyn’s close ally Karie Murphy following the election defeat.

A party source said the ballot was being conducted by Civica, formerly Electoral Reform Services, which has more than 100 years’ experience of conducting elections. They said Civica was “satisfied” with how the process was going.