Union leaders and Democrat politicians on Monday warned the US labour movement against tearing itself apart as eight of the largest unions prepared to leave the national federation in its worst power struggle since the 1930s.

Two groups leading the rebellion, the Teamsters and service workers union, were expected to confirm their formal split from the AFL-CIO late on Monday. Other smaller groups, most of whom are already boycotting the AFL-CIO's 50th anniversary convention in Chicago, are expected to follow suit shortly.