Cosatu has been served with legal papers challenging the suspension of its general secretary Zwelinzima Vavi, its affiliate Numsa said on Monday.

"We can confirm that [the] Food and Allied Workers' Union (Fawu) and SA Football Players' Union (Safpu) are co-applicants to Numsa's court application," National Union of Metalworkers of SA general secretary Irvin Jim said in a statement.

He said they wanted the Congress of SA Trade Unions (Cosatu) to rescind and uplift the "unconstitutional and unprecedented" suspension of Vavi.

Vavi was suspended three weeks ago for bring the federation into disrepute after he admitted to having an affair with a Cosatu employee.

Jim said Numsa had been forced to take legal action as "Cosatu's internal and democratic processes are being undermined and flouted".

He said the decision to legally challenge Cosatu had not been made lightly, and it was not their preferred option of engagement.

"[However]... the sequence of events since the 11th National Congress of Cosatu held in September 2012 shows a clear agenda to have the democratically elected Cosatu general secretary removed."

He said people had called for Vavi's suspension in the February and May central executive committee meetings.

"We now know that Cosatu has been paralysed since September 2012 by the agenda to drive out comrade Vavi."

He said the union would no longer publicly comment on the matter as it was now in the courts.

"We will only comment once court process has been concluded."

Cosatu spokesman Patrick Craven said it was not commenting on the matter at the moment.