David Cameron told MPs at Prime Minister's Questions that Stephen Deans of Unite was a "rogue" operator, who nearly brought the Scottish petrochemical industry to its "knees."
The Prime Minister called on Labour to conduct an inquiry into the saga, following this week's resignation from his job by Mr Deans.
He had been suspended while the site's owner, Ineos, investigated his involvement in the row over the selection of a Labour candidate in the Falkirk constituency.
Management were looking at claims that he used company time for union business. The company threatened to close its petrochemical site but changed its mind after Unite accepted changes to pay, pensions and other terms and conditions.
Mr Cameron said: "Where, frankly, we have a real problem with a rogue trade unionist at Grangemouth who nearly brought the Scottish petrochemical industry to its knees, we need to have a proper inquiry, a Labour inquiry."
Unite general secretary Len McCluskey denounced the comments as a "naked attempt" to gain political advantage from the recent events at Grangemouth.
He said: "The Prime Minister's conduct today was disgraceful. Mr Deans served thousands of working people loyally for 25 years. The tragic truth - which the Prime Minister has no interest in - is that a decent man lost his job trying to protect the jobs of hundreds more.
"The Prime Minister's rush to smear a good and honourable man will appal decent-thinking people. It dishonours the office he holds and he should apologise at once.
"This is nothing more than political attack on trade unions and the working people they are proud to represent, for which David Cameron ought to be thoroughly ashamed."