The Tory-LibDem Coalition is to review laws on industrial intimidation, after allegations over the Unite union's tactics in the Grangemouth dispute.
David Cameron has ordered his government to consider how to tighten up the rules on the issue.
The union has been accused of targeting senior managers from the Ineos chemical company, and their families, in their own homes.
Among the claims is that the children of executives had seen "wanted" posters put through their letter boxes, while their neighbours had been told they were "evil".
The tactics have been defended by Unite leader Len McCluskey who said those out to damage communities could not expect to disappear to their "leafy suburbs".
Mr Cameron called the allegations against Unite "shocking". He also attacked the Labour party for a lack of action on the issue.
During Prime Ministers' Questions in the Commons he urged Mr Miliband to "stand up to the bully boys of Unite" and accused him of "behaving like the mayor of a Sicilian town towards the mafia".
As he hinted at further action on the so-called "leverage" strategy of Unite, he said: "They need to be properly looked at because the party opposite is ducking its responsibilities.
"We will have to consider what we can do to look at this."
But he also appeared to try to appeal to the around three million members of trade unions up and down the country.
"I think there are millions of people in this country who can be very proud of being trade unionists," he said.
"The problem is that they are led so badly by bully-boys, they are led so badly by people who seem to condone intimidating families, intimidating witnesses and intimidating the leader of the Opposition."
A Downing Street source said that the Prime Minister wanted to look at the issue of "industrial intimidation" following the Unite allegations.
The source said: "This was a very deliberate strategy to disrupt the economy and to disrupt people's lives. What they were trying to do is make a very significant intervention in this industrial dispute with Ineos.
"That is the aim of posting things through people's letterboxes and telling their neighbours that they are evil.
"It is a new form of industrial dispute beyond the simple withdrawal of labour, and therefore that could have an effect on communities and the economy. We are considering what we can do about it."
A Labour source said the party had promised an inquiry into the "dinners-for-donors" allegations 18 months ago, adding: "The Prime Minister should stop lecturing the Labour Party and start cleaning up the Conservative Party."
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