THE SNP has launched an investigation after it emerged a party supporter was fined for threatening behaviour towards opponents during the independence referendum campaign.

John Marshall admitted in court to shouting, swearing and uttering a threat in August 25 last year to No campaigners, including a local Labour councillor.

Mr Marshall, 41, was fined £400 at Hamilton Sheriff Court last Friday, where it also emerged he had three previous convictions for breach of the peace.

The SNP said it had made numerous appeals for an end to abusive behaviour during political debate and was investigating the matter.

It said it could not confirm Mr Marshall's membership due to data protection laws but sources within the party have said he is, as well as being a leading Yes campaigner in Holytown, North Lanarkshire.

The Yes Holytown Facebook page has developed a reputation for its robust stance on independence and has demanding the SNP pledges a second referendum in its Scottish Parliament elections manifesto.

There have also been internal clashes within the local SNP branch, with Mossend and Holytown councillor David Baird reported to party headquarters following a row at a recent meeting.

Sources within the branch and two others in Lanarkshire have also recently complained about infiltration by members of Tommy Sheridan's Solidarity grouping.

Mr Marshall could face suspension or even expulsion from the party on the back of the investigation.

An SNP spokesman said: "We are looking into this matter and are investigating. We have made clear on numerous occasions that there is no place for threatening or abusive behaviour in political debate."

According to reports, North Lanarkshire Labour councillor Frank McNally told police Mr Marshall and several others arrived by car after being informed of the campaigning by other Yes supporters.

One of Cllr McNally's team recorded the incident, leading to Mr Marshall being charged.

Fellow Holytown Labour councillor Jimmy Coyle said: “This man went over the top with threatening behaviour towards people simply because they had a a different view from him. That’s not democracy in any form.”