The Conservative Party chairman Grant Shapps has called on Labour leader Ed Miliband to condemn tactics used by the Unite union during the Grangemouth dispute.
It follows the claims of an Ineos manager who said a "mob" was sent by Unite to his home to intimidate him.
Unite said "leverage tactics" were both "legal and legitimate" during an industrial dispute.
It said bad employers should have "nowhere to hide".
But in his letter to Mr Miliband, Mr Shapps described the strategy as "thuggish".
He urged Mr Miliband to condemn the approach, to refuse money from Unite until those responsible were disciplined, and again called on Labour to investigate allegations of vote rigging in the Falkirk constituency.
He claimed Unite, which is the Labour Party's largest donor, sent a group called the Leverage Team to "threaten senior Ineos executives at home, and to seriously intimidate their families".
'Consistent strategy'He said it was "clear from Unite's website" that the tactics were "not an isolated incident" but a "consistent strategy that they have used across the country".
Mr Shapps wrote: "Unite claim that this kind of behaviour is both 'legal and legitimate'. I'm sure you'll agree with me that it falls way short of decency.
"Last year (Unite leader) Len McCluskey praised this as 'a new, sophisticated, smart way to do business'."
He continued: "The Labour Party, and the Scottish Labour Party, backed Unite and their actions throughout the Grangemouth dispute. You must now accept the consequences of this disastrous misjudgement."
Mr Shapps said there were three questions that Mr Miliband, as leader of the Labour Party, must now answer:
- Will you actively condemn this thuggish leverage strategy, and demand that Unite abandon this unacceptable approach?
- Will you now refuse to accept any more money from Unite until those responsible for threatening innocent families are disciplined?
- Will you open a new inquiry to investigate properly the allegations of selection rigging in Falkirk, and accept that Unite attempted to subvert Labour's internal inquiry?
Mr Shapps said if Mr Miliband failed to answer 'yes' to those questions it would "send out the message to the British people that you are too weak to stand up to the union bosses that bankroll your party."
End Quote Unite websiteLeverage is about the democratic right of the Union to ensure that immoral employers cannot hide behind veils of secrecy "
On Unite's website, the union describes leverage as "a process whereby the union commits resources and time to making all interested parties aware of the treatment received by Unite members at the hands of an employer.
"Those interested parties may include shareholders of the employer; competitors of the employer; communities within which the employer operates; customers of the employer and the market place of the employer.
"We will ask those who object to the behaviour of an immoral employer to conduct in lawful protest against the actions of the employer. Where Unite members are involved in such lawful protest the union will use its best endeavours to ensure such members are aware of their rights of lawful protest."
The website said leverage was about the "democratic right of the union to ensure that immoral employers cannot hide behind veils of secrecy and must conduct their business in an open and transparent fashion and accept the consequences of the moral judgements that may follow".
'Legal and legitimate'Unite claimed the tactic had secured "landmark victories" in the past against employers such as Honda, London Buses and Mayr Melnhof Packaging.
A spokesman for the union defended its targeting of Ineos managers during the Grangemouth dispute.
The spokesman added: "All the activities referred to are both legal and legitimate in the context of an industrial dispute. Bad employers should have nowhere to hide.
"Of course all campaigning in the context of the Ineos dispute has now ended with the agreement made with the company.
"However, for the workers and their union to be described as 'bullies' is beyond satire."
Unite also said Mr Shapps' letter showed how "our basic liberties - including the right to protest - are not safe with the Tories" and accused him of "standing up for the powerful against the weak".
On Wednesday, Prime Minister David Cameron blamed a "rogue" union official for the Grangemouth dispute, which almost led to the closure of the petrochemical plant at the site, with the loss of 800 jobs.
Speaking in the Commons, the prime minister said the dispute, which was prompted by the suspension of Unite official Stephen Deans over allegations he was involved in attempting to rig the selection of a Labour candidate in Falkirk, had nearly brought the industry "to its knees".
Mr Cameron called on Labour to hold an inquiry.
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