The prospective Labour parliamentary candidate at the centre of selection-fixing allegations in Falkirk has quit the race.
Labour leader Ed Miliband has turned on the trade union Unite - his biggest financial backer - whose votes helped him become leader.
Tom Watson has announced his resignation from the shadow cabinet, amid a row about the influence of the union Unite on the Labour party.
Shadow health secretary Andy Burnham said he wanted reassurances that that a Government-backed inquiry into the conduct of industrial disputes was not a "political call".
"Of course if there's been intimidation it's unacceptable and that should apply to unions as well as employers," he told BBC One's Sunday Politics show.
"I think I need reassurance that this isn't a political call by Mr Cameron designed to report near the election, so you know we'll see how this develops."
TUC general secretary Frances O'Grady has hit out at the Government's commissioning of an independent review into industrial disputes - and union tactics in particular - saying:
This has nothing to do with good industrial relations and is simply part of the Conservative Party's general election campaign.
We already have some of the most restrictive union laws in the democratic world. Too many Conservatives seem to think that cheap anti-union jibes will detract from the cost of living crisis. The polls suggest they are wrong.
Union officials have dismissed the Government-ordered independent inquiry into industrial disputes as politically motivated and a distraction to the "cost of living crisis".
A Unite spokesman said: "Vince Cable may not have noticed but the Grangemouth dispute has been settled. This review is nothing more than a Tory election stunt which no trade unionist will collaborate with."
GMB general secretary Paul Kenny agreed, saying the announcement "seems like another sop to the Tory backbenches".
He said the "real scandal at Grangemouth" concerned the chairman of the refinery's owners Ineos, Jim Ratcliffe, and how he was "able with impunity to hold the country to ransom".
The joint endorsement of the review into industrial disputes by the Conservative's Francis Maude and the Liberal Democrat's Vince Cable has suggested renewed coalition tensions on the main target of the inquiry.
Mr Maude said the review would chiefly examine: "Allegations about trade union industrial intimidation tactics, including attempts to sabotage businesses supply chains and harass employers' families."
Mr Cable, though, stressed the review would examine wrongdoing on both sides of industrial disputes.
He added: "There are rogue unions but there are also rogue employers, some of whom have in the past engaged in illegal tactics like blacklisting."
Bruce Carr QC's inquiry will examine whether current laws are sufficient to prevent what Government sources described as "inappropriate or intimidatory actions" by trade unions.
The Government-ordered review will examine several areas of industrial disputes, including:
- Underlying causes of industrial relations difficulties in affected industries
- Employer practices, including worker "blacklisting"
- Whether police are empowered to respond to complaints
- Potential impact on the UK's critical national infrastructure
- Consequences for investor confidence in key sectors
Mr Carr, a leading industrial relations lawyer, will head a three-strong panel alongside a representative from both the employers and the unions.
Upon formation, it has six months to gather evidence and file a joint report on recommendations to Cabinet Office Minister Francis Maude and Business Secretary Vince Cable.
Prime Minister David Cameron has ordered a wide-ranging inquiry into industrial disputes, including alleged intimidation tactics of trade unions.
The review, led by Bruce Carr QC, will examine, in particular, claims that "leverage" tactics have been employed by the unions in disputes.
The Government-ordered inquiry follows the bitter industrial dispute that almost led to the closure of the Grangemouth oil refinery in Scotland.
The Unite union dismissed the move as a "Tory election stunt" and said no worker body would "collaborate" with it.
Barclays is to axe 1,700 jobs from across its branch network, according to the Unite union.
The trade union Unite remained Labour's largest donor in the third quarter of 2013, despite its public spat with the party leadership over the selection of an election candidate in Falkirk.
Unite gave Labour £777,740 between July and September, almost a quarter of the party's total donations of £3,157,761, according to official figures.
Conservatives received the most donations in the three-month period, with a total of £3,275,185, while Liberal Democrats took in £98,408, said the Electoral Commission.
Labour launched an internal inquiry in July into allegations that Unite had tried to fix the Falkirk selection for its favoured candidate, but the investigation was closed in September after finding no evidence that rules were breached.
Unite the union is to launch a new credit union, open only members, to take on payday lenders such as Wonga.
“We are determined to wipe out rip-off payday lenders and ensure that they can’t make profits by exploiting victims of the financial downturn," Unite general secretary Len McCluskey said.
"Unite's new credit union service ensures our members can access finance without fear and helps keep them out of the clutches of parasitic lenders."
Credit unions are not-for-profit organisations offering loans, savings and current accounts to their members.
More: Who can join credit unions and how do they work?
One of the unions supporting university staff in their second strike over pay next month has said "enough is enough".
Michael MacNeil, UCU's head of higher education, said: "Staff have suffered year-on-year cuts in the value of their pay and have made it clear that enough is enough.
"We remain committed to trying to resolve this dispute and the employers now have until 3 December to sit down and positively engage with the unions.
"If they don't, then our members and those from our sister unions will be out on strike again, as well as continuing to work to contract."