The second-biggest party in South Africa's coalition government, the Democratic Alliance (DA), has joined the opposition Economic Freedom Fighters (EFF) in seeking to delay a parliamentary vote on the contested 2025 budget on Wednesday.

Most big parliamentary parties are against a proposal in the budget to raise value-added tax by 1 percentage point spread over two years, arguing it would hit the poorest hard.

On Tuesday, a parliamentary committee adopted a report supporting the budget's fiscal framework, despite reservations from many lawmakers on the committee about some of the main revenue-generating measures.

Parliament is due to meet from 14:00 local time on Wednesday to consider the report and vote on the fiscal framework and revenue proposals.

The DA and EFF wrote separate letters to National Assembly Speaker Thoko Didiza saying that Tuesday's proceedings were procedurally flawed.

The EFF's letter said the process was "defective and vulnerable to legal challenge". It asked for the report to be withdrawn and for Wednesday's parliamentary sitting to be rescheduled, threatening legal action if that did not happen.

At Tuesday's parliamentary committee meeting, the biggest party in the governing coalition, the African National Congress (ANC), relied on a small party outside the coalition, ActionSA, to get the support it needed.

The DA has accused it of crossing a "line in the sand" by using support from a party outside the coalition. It also flagged procedural irregularities in its letter to the speaker, local news website News24 reported.

The country's biggest trade union group Cosatu, a longstanding ANC ally, said it would be a "travesty" if parliament's reservations about raising value-added tax and not adjusting personal income tax brackets for inflation were ignored.