By Steve Kingstone BBC News, Sao Paulo

Roberto Jefferson and Lula da Silva are supposed to be allies

The claim was made by the president of the Brazilian Labour Party, a member of the ruling coalition.

Roberto Jefferson said his MPs refused an offer of monthly payments in 2003, but that other parties accepted bribes in return for political loyalty.

The PT said there was "not a grain of truth" in the allegations.

In an interview published in the Folha de Sao Paulo newspaper, Mr Jefferson claimed that members of the Liberal Party (PL) and the Progressive Party (PP) had accepted bribes of up to $US12,000.

He said the financial arrangement only ended in 2005, after he personally brought the matter to the attention of President Luis Inacio Lula da Silva.

The PT said it was "surprised and outraged" at Mr Jefferson's claims and also upheld its broader commitment to fighting corruption.

The Progressive Party denied accepting money in return for political support. The Liberal Party said the claims were lies designed to "blackmail the government".

Investigation

As lawmakers from different parties called on Mr Jefferson to clarify his claims in parliament, supporters of the president were questioning the credibility of the man making the accusations.

Mr Jefferson is himself under investigation, following a corruption scandal at the state-run postal service.

At this stage, his account seems to be based more on hearsay than firm evidence.

Even so, the mere fact that this has surfaced is of great concern for President Lula.

In opinion polls, the president's popularity has been falling.