A Scottish Labour shadow Cabinet member has been called on to resign after he was pictured at a union protest outside a director's house during the toxic Grangemouth dispute.

Drew Smith MSP and other Labour figures were present at the Unite trade union's notorious "leverage" demo at the height of the row with site owners Ineos.

The recent stand-off between Ineos and Unite, which represents many of the workers at Grangemouth, nearly resulted in the plant's closure and the loss of over 1000 jobs.

Initially triggered by the company's investigation of shop steward Stevie Deans, who was also at the centre of the Labour selection debacle in Falkirk, the dispute snowballed and led to the temporary shutdown of the site.

As the war of words intensified, Unite used US-style "leverage" tactics, involving protests outside the homes of senior Ineos figures, to put pressure on the company to back down.

Although Labour leader Ed Miliband distanced himself from the union's strategy, the Sunday Herald can reveal that senior party members were at one of the protests.

In late October - a key point in the dispute - a Unite team turned up outside the Dunfermline home of an Ineos director with an inflatable rat in tow.

Smith, the party's constitution spokesman and chair of Holyrood Labour's trades union group, was one of 13 people pictured. He was standing next to the rat.

Helen Stephen, who is Smith's parliamentary assistant and a member of Labour's Scottish executive committee, was also present.

Michael Sharpe, who is the son of senior Labour MP Cathy Jamieson and another of Smith's aides, was there too. Jamieson is part of shadow chancellor Ed Balls's Treasury team at Westminster.

Both aides were holding placards.

A source close to the MSP said he and his aides were campaigning in the Dunfermline by-election, saw the Unite demonstration and spoke to the protestors briefly.

Smith, a Unite member, was elected in 2011 as a Glasgow list MSP.

Although he is considered to be a rising star, a senior party source criticised Smith's decision: "It is completely irresponsible for Drew Smith to be present at this demonstration. There are huge question marks over his judgment. A trade unionist with any sense would not have gone within 100 miles of that protest."

The leverage tactics used in the Grangemouth dispute were condemned by the Conservatives and led to Prime Minister David Cameron ordering a review of a practice he labelled "shocking".

However, Unite general secretary Len McCluskey defended the union by saying that "bad bosses" should have "nowhere to hide".

On its website, Unite offers a definition of the practice: "Leverage is 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 marketplace of the employer."

Falkirk MP Eric Joyce, who left Labour last year after pleading guilty to assault, has many of the Grangemouth workers in his constituency.

He said: "This image of a Labour shadow cabinet member smiling as he takes part in a leverage squad outside someone's house is thoroughly nauseating. He should resign immediately.

"The Scottish shadow cabinet doesn't feel like a serious prospect at the moment. Members are content to operate at the level of the local councillor which some of them remain."

Murdo Fraser, the Scottish Tories' enterprise spokesman, said: "The disgraceful tactics of Unite the union in picketing the family home of an Ineos director have been widely condemned, but there has not been a word of criticism from Johann Lamont, herself a Unite member.

"Now we know why. This is all a lot closer to home for Johann Lamont and Scottish Labour than they first led us to believe, with senior Labour figures involved including one of her frontbench MSPs.

"Johann Lamont now needs to break her silence, unreservedly condemn this outrageous behaviour, and tell us what disciplinary action she will be taking against Drew Smith and the others involved."

A Scottish Labour spokesman said: "None of our elected members condone or have ever taken part in intimidation."

Smith did not respond to a request for comment.