Jim Murphy launched a broadside attack on Unite boss Len McCluskey as he announced he will resign as leader of the Scottish Labour party.
Mr Murphy said that while trade unionists were "a source of enormous strength and moral purpose" to Labour, the party should not be beholden "to one man."
The head of the Unite union, Len McCluskey, has arrived at the Grangemouth plant to join local union leaders in the talks on the future of the petrochemical plant and oil refinery.
He denied that his appearance was an admission that his union had failed its members over the dispute, insisting he was "here to save the plant."
A new ruling in force from today means that workers will have to pay up to £1,200 to launch a tribunal claim against their employers.
A protest against the ruling, organised by the GMB trade union, will be held outside an employment hearing in Central London today.
Unite leader Len McCluskey described the ruling as taking “a sledgehammer to workers’ rights”.
Supporters of the ruling say that along with a "remission scheme" for those on a low income, it will help stem running costs of the employment tribunal system, which came in at more than £74.4 million last year, at a cost to the taxpayer.