They come from the fusion of two earlier Trotskyist groups the International Group and the Socialist Group. The IG was the part of the older International Marxist Group that remained aligned with the United Secretariat of the Fourth International when part of that group split on a semi-Castroite course, later to become Socialist Action. The Socialist Group was led by Alan Thornett and was a remnant of a larger group which had broken from Gerry Healy's Socialist Labour League in 1973.
The ISG was active within the Labour Party until recently and is now a part of the Socialist Alliance and is generally seen as being close to the SWP. Led now by Alan Thornett it has abandoned its own paper, which has since reappeared as a journal called Socialist Outlook, and helps publish Socialist Resistance with the Socialist Solidarity Network.