Andy Burnham’s Labour leadership campaign has given a heavy hint that it may challenge the outcome of the election if Jeremy Corbyn wins, on the grounds of concerns about infiltrators from other parties.

His campaign director, Michael Dugher, said in a letter to the party’s general secretary, Iain McNicol, that there were still concerns about “entryists” despite the party’s attempts to weed out supporters of other parties.

The letter highlighted “concerns about potential Tory infiltration on a large scale, with a Conservative MP, Conservative media commentators and Conservative councillors all having been rejected.”

It went on: “This suggests the 121,000 registered supporters could include several thousand Tory infiltrators, as well as supporters of other parties seeking to have a vote in the election. While we accept the party has processes in place, there is potentially more that can be done by the party nationally and regionally as well as by local CLPs [constituency Labour parties] to exclude such infiltrators.

“We are also concerned that given the party’s limited resources and the effort required to investigate applicants, this could result in the integrity of the contest being called into question, and the outcome subject to legal challenge.”

Dugher ended by calling for an urgent meeting of all the candidate teams and the party’s procedure committee, warning that leaving it until the next scheduled date of 1 September could “potentially leave insufficient time for the party to act”.

Labour’s handling of the election is now coming under attack from all sides. On social media under the hashtag #LabourPurge there have been complaints that the party is unfairly barring longtime supporters from voting on the grounds they do not back its aims and values.

Matt Beresford, a Labour supporter who is collecting complaints, said about 350 people had contacted him since Thursday morning. He stressed that his findings were not scientific but said the submissions were primarily from Corbyn supporters and about 60 were from members. He said he would be speaking to the Corbyn team – who have asked supporters who have been rejected to contact them – about his findings.

Kate Hoey, the Labour MP for Vauxhall, said there was a danger that “some very unfair decisions are being taken which rule out decent Labour supporters from getting a vote”.

She said she believed there were some cases where personal vendettas meant people were being wrongly barred, and highlighted the case of Jason Cobb, a local community blogger in her area who used to be a member but left during the Blair era.

“His ‘crime’ is that he has drawn attention to some Lambeth council decisions which do not have community support, such as the demolition of the Cressingham estate,” she said. “But his blogs are always perfectly reasonable and indeed I usually agree with them. One or two local councillors really resent his views and I have no doubt that it was one of these Progress-supporting individuals who has flagged his name.

“Today he received his letter saying he didn’t share the Labour party values and therefore was not being allowed to vote … The party has reneged on natural justice with no appeals and we will pay the price when the election is over.”

Others believe the party is engaging in an orchestrated plot to try to rig the result. Jeremy Hardy, a comedian who was excluded after publicly endorsing Green MP Caroline Lucas during the general election campaign, said he believed his exclusion was part of a plot to minimise Corbyn’s chances of winning by inviting the possibility of a legal challenge.

He told BBC Radio 4’s Today programme: “Do you know what, they are so desperate. It wouldn’t surprise me if they are hoping that someone is going to try and actually bring a court case against the Labour party and actually put the whole thing on hold … I think this is just such a mess. They are trying to stop Corbyn, they will do anything.”

Andrew Mackinlay, a former Labour MP, said he thought there could possibly be a concerted attempt to keep Corbyn out, adding: “I think that we would not be having this discussion this morning if all the indications were that [Liz] Kendall or [Yvette] Cooper or Burnham were in the lead.”

The party began its verification process after concerns were raised about infiltrators from other parties signing up to try to influence the results. An initial hunt for “entryists” found Conservative, Green and Socialist Worker party candidates among 1,200 people that the party decided to bar.

The party then intensified its efforts to weed out people who it did not consider supported its aims and values, after an extraordinary surge in new applicants just before the deadline swelled its electorate from 200,000 to 600,000.

Labour has dozens of officials working on trying to spot infiltrators from their comments on social media and two panels of three people making final adjudications, but leaked meeting notes of its procedure committee show the party has been inundated and the system is overstretched.

