Congress of SA Trade Unions (Cosatu) president Sidumo Dlamini must resign, unions sympathetic to the National Union of Metalworkers of SA (Numsa) said on Thursday.

"We are of the firm view that the Cosatu president, as head of the federation, must resign with immediate effect because he no longer enjoys the confidence of workers in the private sector of the SA economy and the public sector," said Moleko Phakedi on behalf of the unions.

"He failed dismally to lead the federation. Cosatu is at its weakest form ever in history. Sidumo cannot provide the leadership needed."

He said Dlamini and some of his national office bearers acted on behalf of their "faction" to the detriment of the unity of the federation.

The unions wanted Numsa to be reinstated unconditionally.

He said the unions had also resolved not to participate in any discussion, formal or informal, until Numsa was reinstated.

Numsa, an ally of Cosatu general secretary Zwelimzima Vavi and Cosatu's biggest affiliate, was expelled from Cosatu earlier this month.

Numsa had taken Cosatu to court over Vavi's suspension for having an affair with a junior employee. Earlier this year the suspension was overturned and Vavi returned to work.