Built up from nothing by publicity-shy venture capitalist Jim Ratcliffe, the group is Britain's biggest private company and is worth billions after aggressively snapping up ailing refining businesses and chemical works run by giants such as BP and ICI.
Ineos operates in 17 countries around the world, but was headquartered in the UK from its founding in 1997 until three years ago when its main office was moved to Switzerland.
The switch was said to have taken place in order to save the firm £406 million over five years, but was heavily criticised by then TUC general secretary Brendan Barber who said at the time that "no UK company with a major domestic presence should be able to cut its tax bill simply by moving a tiny part of its operations overseas".
The criticism was rejected by Tom Crotty, chief executive of Ineos, who said the firm "remained committed to the UK".
Unite general secretary Len McCluskey said this week: "The current uncertainty as to the true nature of the company's UK activities is causing genuine confusion, not least to the Ineos employees we represent, as well as other stakeholders."