Unite is standing behind its "leverage" tactics, which have included sending people to protest outside the home of a director of Ineos, the owner of the Grangemouth petrochemical plant in Scotland.
However, Mario Dunn, a former policy adviser at Unite’s predecessor TGWU, said targeting suppliers and attacking individuals was not a legitimate tactic.
"These are working people doing their jobs and they shouldn’t have to suffer any intimidation. It undermines the rights trade unions believe in," he said.
The affair "inevitably" posed problems for Labour, he said.
"The unfortunate fact is that every time an organisation like Unite does something controversial, given its affiliation to the Labour Party, its political opponents will point to Ed Miliband and say you must take action."
Yesterday David Cameron expressed "shock" at Unite’s tactics while Miliband indicated he would be concerned if reports of Unite’s behaviour were true.
Miliband should step in, according to Gill Morris, chief executive of Connect Communications.
"It will be crucial at Labour’s special conference in March that Ed sets some clear boundaries about what is acceptable and what is not," she said.
The management of PR agency Media Zoo, which handles media relations for Ineos, is adamant that Unite has overstepped the mark.
The agency’s managing director Rachel Pendered expressed dismay over Unite’s description of it as a "child labour PR specialist", which she said was a distortion.
In Dunn’s view singling out Ineos’ PR agency showed "a lack of perspective" from the union.
"It’s not a tactic that rational people would employ," he said.
Unite responded to PRWeek’s request for comment by upping the ante.Unite chief of staff Andrew Murray invited Media Zoo to "enlighten us as to whether it is supplying" a number of emails that appeared in the media over recent days.
Media Zoo director Mark Killick disputed any suggestion that the agency was involved in the leaks.
"We have nothing to do with leaking anything," he said.
Murray added: "Ms Pendered also claims on the Media Zoo website, slightly pretentiously one might feel, that she is 'passionate about beautiful things'… Let her come clean about Media Zoo's activities."
Killick accused Unite of trying to "defend the indefensible".
"It’s intimidating honest people and it’s trying to justify it. It has been rightly slated across the board from the Prime Minister downwards."
The emails in question are connected to the highly politicised drama over the activity of Unite’s official at Grangemouth, Stevie Deans.
Deans was suspended by Ineos over claims he used company time for political campaigning work and his emails were passed to police.
Unite voted for strike action over his treatment, which led to the shutdown of the oil refinery and petrochemical plant.
The union backed down after Ineos said it would close the petrochemical plant, leading to Ineos reversing its decision.
On Monday Deans, who has also faced questions over undue influence on the selection of his local Labour parliamentary candidate, resigned from Ineos.
Ineos had been expected to reveal the outcome of the disciplinary case against him on Tuesday.
Stephen Day, managing director of Ineos’ public affairs agency Portcullis, which has helped Ineos explain its stance to MPs and MSPs, complained Unite behaviour such as calling Ineos chairman Jim Ratcliffe a rat and writing to Ineos customers, banks and suppliers was "unacceptable and irresponsible".
Unite’s comms chief rejected suggestions that Unite had been too aggressive.
"In these situations we have members to defend. They are our primary responsibility, and they want to see their union fighting on their behalf," said Pauline Doyle, director of campaigns and comms.
"But after a dispute we work to restore morale and rebuild industrial relations. This has been impossible because of the onslaught from the right-wing press, egged on we believe by judicious leaking of emails from Media Zoo."
The organisation regards the leverage strategy as necessary when it cannot persuade an employer to engage by other means, and believes people have a right to protest.
However, there are signs that not all of its members appreciate Unite’s style of fighting.
Jonathan Roberts last night went public to say he had resigned after seven years.
"My membership of an organisation that behaves in such a depressing, devious and combative way reflects badly upon me as a person," he said.
Comms chiefs at the Communication Workers Union, Unison and the GMB (which represents many workers dependent on Grangemouth for contracts) declined to comment when asked if they felt Unite had been too aggressive.