Image caption Unite members have targeted the homes of several Ineos managers, including this one in Dunfermline

The prime minister has described allegations that Unite union members targeted the homes of Grangemouth refinery managers as "quite shocking".

The row follows claims by an Ineos director that a "mob" was sent by Unite to his home to intimidate him during the recent dispute.

David Cameron called on the Labour Party to investigate the claims about the union, which is its largest donor.

Unite insisted all its activities have been legal and legitimate.

The union defended its use of so-called leverage tactics, where managers are directly targeted as part of a protest, and argued that bad employers should have "nowhere to hide".

Analysis It was a bitter dispute set against the backdrop of heavy industry. But it's become clear that the Battle of Grangemouth was also fought in the leafy suburbs of Fife and Hampshire. The BBC understands that four Ineos directors were targeted by protestors from Unite, a tactic the union calls "leverage". Others call it "intimidation". In a quiet corner of Dunfermline today neither the Ineos director who lives here, nor his neighbours, wanted to be interviewed, concerned about their security. But the reactions of people here ranged from bemusement to shock and anger. One woman told me that she thought the protest was "disgusting". There had been children around, she said. In this smart suburb of Dunfermline, this feels a little like class war.

It comes after an Ineos director, who is believed to live in Dunfermline, told the Daily Mail how he feared for the safety of his wife and his two young children after 30 Unite protesters turned up at his home during the school holidays.

Police Scotland confirmed officers were called to an address in Dunfermline on Friday 18 October following reports of a protest.

A spokesman said: "The occupants were not present during the protest and demonstrators had dispersed prior to their return to the property. Inquiries into this matter are ongoing."

BBC Scotland understands several other Ineos executives, including chairman and founder Jim Ratcliffe, were targeted by Unite.

It is understood from police sources that there were four Unite actions against Ineos staff in Hampshire in October.

One was at a hotel in Beaulieu; another at a yacht club in Lymington. Ineos's headquarters is at Lyndhurst, in Hampshire, and senior staff are thought to live in the area.

Speaking at a Cameron Direct event in Wolverhampton, the prime minister said the claims should be properly investigated by the Labour Party.

Mr Cameron said people had a right to protest, and trade unions had a right to represent their members and take industrial action.

I don't know the truth of these allegations but I condemn intimidatory tactics from either unions or management in any dispute Ed Miliband, Labour leader

But he said: "No-one has a right to intimidate. Nobody has a right to bully. Nobody has a right to threatened people's families. No-one has a right to threaten people in their homes. If these things have happened, it is very serious. It needs to be properly examined.

"We shouldn't forget that the union action at Grangemouth was in danger of killing off an absolutely vital industry for Scotland and for the UK.

"So, I think we need to have these actions properly investigated and I think the Labour Party does also need to investigate what has been happening in the Unite union, not least because the person in question (Unite official Stephen Deans, whose treatment by Ineos sparked the dispute) is still chairman of the Falkirk Labour Party.

"So these actions need to be taken. It's not acceptable to intimidate people."

Labour leader Ed Miliband told BBC Radio Bristol he was "against any intimidatory tactics in any situation and I think that's the most important thing".

He added: "Let me be clear about this. I don't know the truth of these allegations but I condemn intimidatory tactics from either unions or management in any dispute.

"I think what David Cameron should be doing though is making sure is that in this dispute we've now got people back to work, and that is really important because we need to protect that national asset of Grangemouth and need to protect those jobs and we do that by both sides working together."

The Conservative Party chairman Grant Shapps had earlier called on Mr Miliband to condemn Unite's tactics during the Grangemouth dispute.

In his letter to Mr Miliband, Mr Shapps described the strategy as "thuggish".

Analysis The Grangemouth dispute and the Labour selection row in Falkirk are two separate controversies which are inextricably linked. Claims that Unite sought to fix the selection battle, so that its preferred candidate became the next Falkirk MP, forced Ed Miliband to overhaul his party's financial links with affiliated unions. When the original claims surfaced, the prime minister said it was a sign Len McCluskey was pulling Labour's strings. Unite's role in the near-closure of the Grangemouth petrochemical plant over the treatment of a shop steward involved in the Falkirk row has given Labour's opponents yet more ammunition. Allegations of dirty tricks by a union which many believe almost cost 800 Scottish workers their jobs adds fuel to a very political fire. While Unite denies wrongdoing, the latest development makes life for Labour and its links to Britain's largest union yet more uncomfortable.

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 sent a group called the Leverage Team to "threaten senior Ineos executives at home, and to seriously intimidate their families".

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".

Unite claims on its website that leveraging tactics have 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."

'Rogue official'

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".

Media playback is unsupported on your device Media caption General Secretary of Unite, Len McCluskey: "Nothing illegal, and certainly no bullying or intimidation"

The Grangemouth dispute began over the treatment of Unite union official, Stephen Deans, over allegations he was involved in attempting to rig the selection of a Labour candidate in Falkirk.

It escalated to the threat of strike action but despite this being dropped, the operator shut down the plant and issued a "survival plan", which was rejected by union members.

Ineos then announced the closure of the petrochemical plant at the site with the loss of 800 jobs.

After crisis talks, the union accepted the revised terms and conditions, allowing the plant to stay open.

On Wednesday, the prime minister blamed a "rogue" union official for the Grangemouth dispute.

Speaking in the Commons, Mr Cameron said the dispute had nearly brought the industry "to its knees".

Unite general secretary Len McCluskey has since challenged Mr Cameron to repeat his "rogue official" allegation outside of the Commons, "to see what the courts make of them."