Johannesburg - Cosatu congratulated the ANC on Friday on its election victory, but warned against complacency by its alliance partner.
With most votes counted by Friday afternoon, the African National Congress was leading the national elections and eight of the nine provincial elections, Congress of SA Trade Unions spokesperson Patrick Craven said in a statement.
"The ANC has proved again that it is still the party in which the majority of South Africans, especially the working class and the poor, put their trust."
According to preliminary results the party had 11 332 698 votes by 17:30 - 62.23% of the national vote.
This was after 99.53% of counting had been completed.
Craven said: "The election has also confirmed the absence of any credible opposition party which offers an alternative which could appeal to workers."
The trade union federation warned that the ruling party should not become complacent nor take voters for granted, given that the party's support in the election dropped slightly from the previous general election in 2009.
"The big majority of the working class and the poor have remained loyal to their traditional party, but, like Cosatu itself, they have not given it a blank cheque.
"They will be looking for evidence that there is now going to be a real and substantial improvement in their lives and that the crisis of unemployment, poverty and unemployment is going to be resolved," Craven said.
He said Cosatu members played a key role in achieving the high voter turnout and securing victory for the ANC.
Cosatu's biggest affiliate, the National Union of Metalworkers of SA, had called on its members not to support the ANC in the elections. This has recently been a bone of contention between the union and the federation.
The union federation encouraged the ANC to tackle unemployment, poverty and inequality during its next term in government.
Cosatu demanded measures - including strategic nationalisation and state ownership of strategic sectors of the economy, a radical overhaul in macroeconomic policy - to help realise the ideals set out in the Freedom Charter.