By Martin Plaut BBC World Service

The South African trade union movement starts a two day strike against government privatisation policy on Tuesday.

The Congress of South African Trades Unions, Cosatu, is allied to the ruling African National Congress but objects strongly to the privatisation of state owned enterprises.

The strike is the latest open example of conflict over policy between the government of President Thabo Mbeki and some of its closest supporters.

Will the president see red over Cosatu strike ?

Many in South Africa now ask how long the alliance, which includes the South African Communist Party, can last.

Ignoring the poor

The African National Congress wears its credentials as a liberation movement with great pride.

Nothing is more insulting to it than to suggest that it is somehow less than caring about the poorest in South African society.

This is why it has reacted so sharply against the two day general strike - warning against what is termed "ultra-leftist" principles.

Poverty is still rife among black South Africans

In reality the urban working class has been moving away from the ANC for some time.

The privatisation of electricity or telecommunications may please foreign investors, but it has cost ordinary workers many thousands of jobs.

The trades unions, who are in a political alliance with the ANC, could not ignore this challenge to their interests.

Numerous meetings have failed to bridge the gap.

And now, in conjunction with the South African Communist Party, they are taking their protests to the streets.

Political commentators now suggest that the alliance with the ANC may be at breaking point. Some even talk of the founding of a separate party of labour.

But so far union leaders have resisted a final separation, and have managed the anger of their supporters, rather than challenging the government's right to rule.